[libcxx] [libcxxabi] [libunwind] [llvm] [runtimes] Probe for -nostdlib++ and -nostdinc++ with the C compiler (PR #108357)

[Louis Dionne via llvm-commits]

================
@@ -38,9 +38,13 @@ check_cxx_compiler_flag(-nolibc CXX_SUPPORTS_NOLIBC_FLAG)
 # required during compilation (which has the -nostdlib++ or -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
+#
+# Adding flags to CMAKE_REQUIRED_FLAGS will include the flags both when testing
+# compilation of C and C++. Therefore test to make sure that the flags are
+# supported by the C compiler driver, before deciding to include them.
 
-check_cxx_compiler_flag(-nostdlib++ CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+check_c_compiler_flag(-nostdlib++ C_SUPPORTS_NOSTDLIBXX_FLAG)
----------------
ldionne wrote:

Do we compile any C sources in libc++? Why do we care what flags the C compiler supports for libc++ and libc++abi? (I understand why it matters for libunwind where we do have C sources).

The reason I'm asking is that there are some simplifications we can make if we assume that all supported compilers accept `-nostdlib++`, and this patch makes it impossible to make that assumption.

https://github.com/llvm/llvm-project/pull/108357