Implement used_on_fn_def

[carbotaniuman]

This lets one place the #[used] attribute on function definitions. This is useful when writing
a functions as a library to be linked in later, perhaps against some C code. #94348

I'm not sure if my changes to contains_extern_indicator are correct,
they appear to be doing the right thing and letting the functions through to
LLVM, but I'm not sure if they're doing too much.

I'm also completely lost at what to do for the gcc codegen backend.

This also still needs the lint for being placed on a generic function (unless that should just error),
and I'm not sure if no_mangle should also add to used_globals.

[rust-highfive]

Some changes occured to rustc_codegen_gcc

cc @antoyo

[rust-highfive]

r? @jackh726

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.debug-assertions := True
configure: rust.overflow-checks := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
   Compiling tidy v0.1.0 (/checkout/src/tools/tidy)
    Finished release [optimized] target(s) in 8.64s
tidy check
tidy: Skipping binary file check, read-only filesystem
tidy error: following path contains more than 985 entries, you should move the test to some relevant subdirectory (current: 986): /checkout/src/test/ui
* 629 error codes
* highest error code: E0787
Found 504 error codes
Found 0 error(s) in error codes
Found 0 error(s) in error codes
Done!
tidy error: /checkout/src/test/codegen/used_on_fn_def.rs: too many trailing newlines (2)
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: Zero }', src/tools/tidy/src/features.rs:387:32
Build completed unsuccessfully in 0:00:11

[bjorn3]

This is useful when writing a functions as a library to be linked in later, perhaps against some C code.

Why is #[no_mangle] not enough? That should cause the function to be exported from the linked library.