[compiler-rt] [compiler-rt] Fix detection of warnings flags (PR #125602)

[Eli Schwartz via llvm-commits]

eli-schwartz wrote:

```meson
project('testflags', 'c')

cc = meson.get_compiler('c')
args = cc.get_supported_arguments('-Wno-forgotten-towel', '-Wno-pednatic', '-Wthingy',
                                  '-Wbuiltin-declaration-mismatch', '-flto')
message('supported args', args)
```

```
$ CC=gcc meson setup --wipe builddir
Compiler for C supports arguments -Wno-forgotten-towel: NO 
Compiler for C supports arguments -Wno-pednatic: NO 
Compiler for C supports arguments -Wthingy: NO 
Compiler for C supports arguments -Wbuiltin-declaration-mismatch: YES 
Compiler for C supports arguments -flto: YES 
Message: supported args ['-Wbuiltin-declaration-mismatch', '-flto']
```

```
$ CC=clang
Compiler for C supports arguments -Wno-forgotten-towel: NO 
Compiler for C supports arguments -Wno-pednatic: NO 
Compiler for C supports arguments -Wthingy: NO 
Compiler for C supports arguments -Wbuiltin-declaration-mismatch: NO 
Compiler for C supports arguments -flto: YES 
Message: supported args ['-flto']
```

Was interested to see how this gets handled, as I had a dim memory that meson went to some lengths to work around compiler-specific weirdnesses here.

It turns out that for:
- clang, meson is automatically passing `-Werror=unknown-warning-option -Werror=unused-command-line-argument -Werror=ignored-optimization-argument` to all "check supported flags" checks
- both GCC and clang, but specifically intended for the sake of GCC, meson is automatically passing both -Wxxx and -Wno-xxx any time you use an argument of the form -Wno-*, since GCC silently ignores unknown -Wno-xxx


I wonder, if it would be a good idea to instead have https://github.com/llvm/llvm-project/blob/d810c741ad7a5f5bee8fe833b9ee4023446dd39c/compiler-rt/cmake/Modules/BuiltinTests.cmake#L112-L123 automatically handle this for you.

https://github.com/llvm/llvm-project/pull/125602