Add a run-make test that checks our `core::ffi::c_*` types against Clang

[tgross35]

It would be good to have a test that our C interop types are always compatible with C (c_char, c_long, c_longlong all vary to some extent). This will more or less be what @taiki-e did at #129945, just rolled into a run-make test rather than a shell script.

The test will basically need to do the following:

1. Loop through each target string in the output of rustc --print target-list

2. Map the rust target to a LLVM target if they aren't the same

3. Loop through a list of each c_* type available in core::ffi

4. Query clang -E -dM -x c /dev/null -target LLVM_TARGET to get a list of builtin definitions. Of note will be something like the following (comments added by me)

   #define __CHAR_BIT__ 8 // c_char
   #define __CHAR_UNSIGNED__ 1 // char signedness
   
   #define __SIZEOF_DOUBLE__ 8 // c_double
   #define __SIZEOF_FLOAT__ 4 // c_float
   #define __SIZEOF_INT__ 4 // c_int
   #define __SIZEOF_LONG_DOUBLE__ 16 // c_longdouble
   #define __SIZEOF_LONG_LONG__ 8 // c_longlong
   #define __SIZEOF_LONG__ 4 // c_long
   #define __SIZEOF_POINTER__ 4 // *const c_void
   #define __SIZEOF_PTRDIFF_T__ 4 // c_ptrdiff_t
   #define __SIZEOF_SHORT__ 2 // c_short
   #define __SIZEOF_SIZE_T__ 4 // c_size_t
   
   #define __BOOL_WIDTH__ 8 // bool
   #define __INTPTR_WIDTH__ 32 // isize
   #define __INT_WIDTH__ 32 // c_int
   #define __LLONG_WIDTH__ 64 // c_longlong
   #define __LONG_WIDTH__ 32 // c_long
   #define __POINTER_WIDTH__ 32 // *const c_void
   #define __PTRDIFF_WIDTH__ 32 // c_ptrdiff_t
   #define __SHRT_WIDTH__ 16 // c_short
   #define __SIZE_WIDTH__ 32 // c_size_t
   #define __UINTPTR_WIDTH__ 32 // usize

5. Use the above to construct a simple Rust program that can verify the sizes and (when applicable) signedness line up at compile time. Probably do an assignment that will catch mismatched types plus a const assert for anything that doesn't have literals.

   // input to check `c_short`
   const C_SHORT: u{c_sizeof_short} = 0;
   const RUST_SHORT: core::ffi::c_short = C_SHORT;
   
   ```rust
   // input to check pointer width
   const _: () = assert!(size_of::<*const core::ffi::c_void>() * 8 == {c_pointer_width});

6. Run rustc -Z no-codegen and check success/failure against a list of xfail targets.

run_make_support has clang available, example usage: https://github.com/rust-lang/rust/blob/e84902d35a4d3039c794e139eb12fba3624c5ff1/tests/run-make/cross-lang-lto-clang/rmake.rs. There is probably some way we could check this on MSVC targets too.

[tgross35]

One tricky thing is that this would need to build-std for every target in CI, which is unfortunate. It might be better to have the test do something like copy core/ffi/mod.rs.html and preprocess to remove everything besides type aliases, then test against that rather than core::ffi (making the compiled file #![no_core]).

[jieyouxu]

Remark: I don't think we would want to -Z build-std for every target in CI because that sounds like it's going to take a while

[tgross35]

Remark: I don't think we would want to -Z build-std for every target in CI because that sounds like it's going to take a while

Yeah, I realized that limitation after writing this. Just copying core::ffi to its own crate seems reasonable enough to avoid this since it's a pretty simple module - do we do this for any other test? (i.e. use the library by path rather than the built version).