[clang] [compiler-rt] [libcxx] [libunwind] [llvm] [openmp] [cmake] switch to CMake's native `check_{compiler,linker}_flag` (PR #96171)

[Ryan Prichard via llvm-commits]

rprichard wrote:

This check in libcxx/cmake/config-ix.cmake seems to break:
```
check_cxx_compiler_flag(-nostdlib++ CXX_SUPPORTS_NOSTDLIBXX_FLAG)
```

It appears that it isn't passing either `-nostdlib++` or `-lc++` to the clang linker command-line, but then `-lc++` _is_ passed to `ld.lld` because `-nostdlib++` is missing. The check fails because libc++ hasn't been built yet.

It seems that the older function had been dealing with this:

```
-function(llvm_check_compiler_linker_flag lang flag out_var)
-  # If testing a flag with check_c_compiler_flag, it gets added to the compile
-  # command only, but not to the linker command in that test. If the flag
-  # is vital for linking to succeed, the test would fail even if it would
-  # have succeeded if it was included on both commands.
-  #
-  # Therefore, try adding the flag to CMAKE_REQUIRED_FLAGS, which gets
-  # added to both compiling and linking commands in the tests.
...
```

It sounds like the right fix is to use `check_linker_flag` though? I can test that...



https://github.com/llvm/llvm-project/pull/96171