`-Cllvm-args` usability improvement

fixes: rust-lang#26338
fixes: rust-lang#115564

Two problems were found during playing with `-Cllvm-args`

1. When `llvm.link-shared` is set to `false` in `config.toml`, output of `rustc -C llvm-args='--help-list-hidden'` doesn't contain `--emit-dwarf-unwind` and `--emulated-tls`. When it is set to `true`, `rustc -C llvm-args='--help-list-hidden'` emits `--emit-dwarf-unwind`, but `--emulated-tls` is still missing.
2. Setting `-Cllvm-args=--emit-dwarf-unwind=always` doesn't take any effect, but `-Cllvm-args=-machine-outliner-reruns=3` does work.

### 1

Adding `RegisterCodeGenFlags` to register codegen flags fixed the first problem. `rustc -C llvm-args='--help-list-hidden'` emits full codegen flags including `--emit-dwarf-unwind` and `--emulated-tls`.

### 2

Constructing `TargetOptions` from `InitTargetOptionsFromCodeGenFlags` in `LLVMRustCreateTargetMachine` fixed the second problem. The `LLVMRustSetLLVMOptions` calls `ParseCommandLineOptions` which parses given `llvm-args`. For options like `machine-outliner-reruns`, it just works, since the codegen logic directly consumes the parsing result:

[machine-outliner-reruns register](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/lib/CodeGen/MachineOutliner.cpp#L114)
[machine-outliner-reruns consumption](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/lib/CodeGen/MachineOutliner.cpp#L1138)

But for flags defined in `TargetOptions` and `MCTargetOptions` to take effect, constructing them with `InitTargetOptionsFromCodeGenFlags` is essential, or the parsing result is just not consumed. Similar patterns can be observed in [lli](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/tools/llc/llc.cpp#L494), [llc](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/tools/lli/lli.cpp#L517), etc.

…llaumeGomez

Rollup of 8 pull requests

Successful merges:

 - rust-lang#115345 (MCP661: Move wasm32-wasi-preview1-threads target to Tier 2)
 - rust-lang#115604 (rustdoc: Render private fields in tuple struct as `/* private fields */`)
 - rust-lang#115624 (Print the path of a return-position impl trait in trait when `return_type_notation` is enabled)
 - rust-lang#115629 (Don't suggest dereferencing to unsized type)
 - rust-lang#115633 (Lint node for `PRIVATE_BOUNDS`/`PRIVATE_INTERFACES` is the item which names the private type)
 - rust-lang#115634 (Use `newtype_index` for `IntVid` and `FloatVid`.)
 - rust-lang#115638 (`-Cllvm-args` usability improvement)
 - rust-lang#115649 (diagnostics: add test case for trait bounds diagnostic)

r? `@ghost`
`@rustbot` modify labels: rollup

…llaumeGomez

Rollup of 8 pull requests

Successful merges:

 - rust-lang#115345 (MCP661: Move wasm32-wasi-preview1-threads target to Tier 2)
 - rust-lang#115604 (rustdoc: Render private fields in tuple struct as `/* private fields */`)
 - rust-lang#115624 (Print the path of a return-position impl trait in trait when `return_type_notation` is enabled)
 - rust-lang#115629 (Don't suggest dereferencing to unsized type)
 - rust-lang#115633 (Lint node for `PRIVATE_BOUNDS`/`PRIVATE_INTERFACES` is the item which names the private type)
 - rust-lang#115634 (Use `newtype_index` for `IntVid` and `FloatVid`.)
 - rust-lang#115638 (`-Cllvm-args` usability improvement)
 - rust-lang#115649 (diagnostics: add test case for trait bounds diagnostic)

r? `@ghost`
`@rustbot` modify labels: rollup

…llaumeGomez

Rollup of 6 pull requests

Successful merges:

 - rust-lang#104299 (Clarify stability guarantee for lifetimes in enum discriminants)
 - rust-lang#115088 (Fix Step Skipping Caused by Using the `--exclude` Option)
 - rust-lang#115201 (rustdoc: list matching impls on type aliases)
 - rust-lang#115633 (Lint node for `PRIVATE_BOUNDS`/`PRIVATE_INTERFACES` is the item which names the private type)
 - rust-lang#115638 (`-Cllvm-args` usability improvement)
 - rust-lang#115643 (fix: return early when has tainted in mir-lint)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#115638 - ldm0:ldm/llvm-args-fix, r=nikic

`-Cllvm-args` usability improvement

fixes: rust-lang#26338
fixes: rust-lang#115564

Two problems were found during playing with `-Cllvm-args`

1. When `llvm.link-shared` is set to `false` in `config.toml`, output of `rustc -C llvm-args='--help-list-hidden'` doesn't contain `--emit-dwarf-unwind` and `--emulated-tls`. When it is set to `true`, `rustc -C llvm-args='--help-list-hidden'` emits `--emit-dwarf-unwind`, but `--emulated-tls` is still missing.
2. Setting `-Cllvm-args=--emit-dwarf-unwind=always` doesn't take any effect, but `-Cllvm-args=-machine-outliner-reruns=3` does work.

### 1

Adding `RegisterCodeGenFlags` to register codegen flags fixed the first problem. `rustc -C llvm-args='--help-list-hidden'` emits full codegen flags including `--emit-dwarf-unwind` and `--emulated-tls`.

### 2

Constructing `TargetOptions` from `InitTargetOptionsFromCodeGenFlags` in `LLVMRustCreateTargetMachine` fixed the second problem. The `LLVMRustSetLLVMOptions` calls `ParseCommandLineOptions` which parses given `llvm-args`. For options like `machine-outliner-reruns`, it just works, since the codegen logic directly consumes the parsing result:

[machine-outliner-reruns register](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/lib/CodeGen/MachineOutliner.cpp#L114)
[machine-outliner-reruns consumption](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/lib/CodeGen/MachineOutliner.cpp#L1138)

But for flags defined in `TargetOptions` and `MCTargetOptions` to take effect, constructing them with `InitTargetOptionsFromCodeGenFlags` is essential, or the parsing result is just not consumed. Similar patterns can be observed in [lli](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/tools/llc/llc.cpp#L494), [llc](https://github.com/rust-lang/llvm-project/blob/0537f6354cffe546cbf47f6dc9c7f82e49e86cfb/llvm/tools/lli/lli.cpp#L517), etc.