LLVM 7 is over a year old, which should be plenty for compatibility. The
last LLVM 6 holdout was llvm-emscripten, which went away in rust-lang#65501.

I've also included a fix for LLVM 8 lacking `MemorySanitizerOptions`,
which was broken by rust-lang#66522.

For SGX, the relocation using the relocation table is done by
the code in rust/src/libstd/sys/sgx/abi/reloc.rs and this code
should not require relocation. Setting RelaxELFRelocations flag
if allows this to happen, hence adding a Target Option for it.

Fixes rust-lang#66787

This commit updates the `wasi` crate used by the standard library which
is used to implement most of the functionality of libstd on the
`wasm32-wasi` target. This update comes with a brand new crate structure
in the `wasi` crate which caused quite a few changes for the wasi target
here, but it also comes with a significant change to where the
functionality is coming from.

The WASI specification is organized into "snapshots" and a new snapshot
happened recently, so the WASI APIs themselves have changed since the
previous revision. This had only minor impact on the public facing
surface area of libstd, only changing on `u32` to a `u64` in an unstable
API. The actual source for all of these types and such, however, is now
coming from the `wasi_preview_snapshot1` module instead of the
`wasi_unstable` module like before. This means that any implementors
generating binaries will need to ensure that their embedding environment
handles the `wasi_preview_snapshot1` module.

This commit breaks early-lint registration, which will be fixed in the
next commit. This movement will allow essentially all crates in the compiler
tree to declare lints (though not lint passes).

Since it's just a type alias this isn't too difficult and once Session
is moved back we can make this be the canonical location.

Update the `wasi` crate for `wasm32-wasi`

This commit updates the `wasi` crate used by the standard library which
is used to implement most of the functionality of libstd on the
`wasm32-wasi` target. This update comes with a brand new crate structure
in the `wasi` crate which caused quite a few changes for the wasi target
here, but it also comes with a significant change to where the
functionality is coming from.

The WASI specification is organized into "snapshots" and a new snapshot
happened recently, so the WASI APIs themselves have changed since the
previous revision. This had only minor impact on the public facing
surface area of libstd, only changing on `u32` to a `u64` in an unstable
API. The actual source for all of these types and such, however, is now
coming from the `wasi_preview_snapshot1` module instead of the
`wasi_unstable` module like before. This means that any implementors
generating binaries will need to ensure that their embedding environment
handles the `wasi_preview_snapshot1` module.

…ntril

Move Sessions into (new) librustc_session

This PR moves `ParseSess` and `Session` from their current locations into a new crate, `librustc_session`.

There are several intents behind this change. librustc is a very large crate, and we want to split it up over time -- this movement removes the sizeable session module from it. It also helps allow for future movement of things not coupled to TyCtxt but coupled to Session out of the crate.

This movement allows allows for a future follow-up PR which unifies Session and ParseSess, allowing for a single source of truth for APIs interested in global options throughout the compiler; the ParseSess is already created directly as a member of Session in the current compiler (i.e., we do not first construct a ParseSess and then move it into Session later in the compilation).

This PR intentionally avoids changing numerous imports throughout the tree to new locations of the moved types; this is needless noise and can be done as needed.

In the process of moving the sessions back, the lint system received an update as well -- notably, early buffered lints are no longer ad-hoc declared as enum pairs and later associated with proper lint declarations. They are still separately handled (buffered), it is a little unclear whether this is truly necessary, but regardless is left for future PRs.

Many of the types moved back are sort of ad-hoc placed into the same crate (librustc_session) instead of creating other crates; it's unclear whether this is actually a good thing, but it seemed better than creating numerous tiny crates which served no purpose on their own.

parse_enum_item -> parse_enum_variant

r? @estebank

miri: add throw_machine_stop macro

r? @oli-obk
This helps Miri: rust-lang/miri#1093

…xcrichton

Change Linker for x86_64-fortanix-unknown-sgx target to rust-lld

Changed linker for `x86_64-fortanix-unknown-sgx` target to `rust-lld`
This change needed the RelaxELFRelocations flag to be set for it to work correctly

r? @jethrogb

…obk,RalfJung

[const-prop] Fix ICE calculating enum discriminant

Fixes rust-lang#66787

Different approach than rust-lang#66857

r? @oli-obk
cc @RalfJung @eddyb

Update the minimum external LLVM to 7

LLVM 7 is over a year old, which should be plenty for compatibility. The
last LLVM 6 holdout was llvm-emscripten, which went away in rust-lang#65501.

I've also included a fix for LLVM 8 lacking `MemorySanitizerOptions`,
which was broken by rust-lang#66522.