Special-case `.llvm` in mangler

Fixes rust-lang#60925 and fixes rust-lang#53912.

r? @michaelwoerister
cc @eddyb

Rollup of 9 pull requests

Successful merges:

 - #60742 (Allow const parameters in array sizes to be unified)
 - #60756 (Add better tests for hidden lifetimes in impl trait)
 - #60928 (Changes the type `mir::Mir` into `mir::Body`)
 - #61024 (tests: Centralize proc macros commonly used for testing)
 - #61157 (BufReader: In Seek impl, remove extra discard_buffer call)
 - #61195 (Special-case `.llvm` in mangler)
 - #61202 (Print PermissionExt::mode() in octal in Documentation Examples)
 - #61259 (Mailmap fixes)
 - #61273 (mention that MaybeUninit is a bit like Option)

Failed merges:

r? @ghost

This changes a mistake introduced in rust-lang#61195 where the mangling
workaround used was incorrect.

…r=eddyb

codegen: change `$6d$` to `$u6d$`

This changes a mistake introduced in rust-lang#61195 where the mangling
workaround used was incorrect, resolving [this comment](rust-lang#61195 (comment)) from @eddyb.

r? @eddyb

…gjubilee

cg_llvm: Remove the `mod llvm_` hack, which should no longer be necessary

This re-export was introduced in rust-lang@c76fc3d, as a workaround for rust-lang#53912.

In short, there was/is an assumption in some LLVM LTO code that symbol names would not contain `.llvm.`, but legacy symbol mangling would naturally produce that sequence for symbols in a module named `llvm`.

This was later “fixed” by adding a special case to the legacy symbol mangler in rust-lang#61195, which detects the sequence `llvm` and emits the `m` in an escaped form. As a result, there should no longer be any need to avoid the module name `llvm` in the compiler itself.

(Symbol mangling v0 avoids this problem by not using `.` in the first place, outside of the “vendor-specific suffix”.)

Rollup merge of rust-lang#136611 - Zalathar:llvm-underscore, r=workingjubilee

cg_llvm: Remove the `mod llvm_` hack, which should no longer be necessary

This re-export was introduced in rust-lang@c76fc3d, as a workaround for rust-lang#53912.

In short, there was/is an assumption in some LLVM LTO code that symbol names would not contain `.llvm.`, but legacy symbol mangling would naturally produce that sequence for symbols in a module named `llvm`.

This was later “fixed” by adding a special case to the legacy symbol mangler in rust-lang#61195, which detects the sequence `llvm` and emits the `m` in an escaped form. As a result, there should no longer be any need to avoid the module name `llvm` in the compiler itself.

(Symbol mangling v0 avoids this problem by not using `.` in the first place, outside of the “vendor-specific suffix”.)