These types are currently rejected for `as` casts by the compiler.
Remove this incorrect check and add codegen tests for all conversions
involving these types.

…ligation causes inference side-effects

Don't ICE because recomputing overflow goals during find_best_leaf_obligation causes inference side-effects

See the comments for more info. Reprocessing overflowed obligations may cause *other* goals to go from ambig -> pass/fail, causing an ICE. This suppresses that error, but makes all the overflow obligations messages back to their root obl. That kinda sucks, but 🤷

Fixes rust-lang#124834
Fixes rust-lang#124845

r? lcnr

Update linker-plugin-lto.md to include LLVM 18

I did this manually not via provided script.

…Gomez

rustdoc-json-types: Document `Id`

Inspired by finding [someone looking into the internals of an id](rust-lang#91609 (comment)), I realized we only document the fact that these are opaque on [the rfc](https://rust-lang.github.io/rfcs/2963-rustdoc-json.html#id).

r? `@GuillaumeGomez`

…=lcnr

Uplift `FnSig` into `rustc_type_ir` (redux)

Since the last one got so messed up with `try` build.

rust-lang#125157 (comment)

r? lcnr

…mpiler-errors

Fix assertion when attempting to convert `f16` and `f128` with `as`

These types are currently rejected for `as` casts by the compiler. Remove this incorrect check and add codegen tests for all conversions involving these types.