[clang] suppress warnings from macros in system headers

[CendioOssman]

clang complains when using poorly written macros in system headers in a way that gcc doesn't, which is highly problematic as we try to use -Werror.

E.g. this code:

#include <winsock2.h>

int main(int argc, char** argv)
{
    int fd = 12;
    fd_set fds;

    FD_ZERO(&fds);
    FD_SET(fd, &fds);
    
    return 0;
}

Will cause this error on Windows:

fdset.c:9:5: warning: comparison of integers of different signs: 'SOCKET' (aka 'unsigned long long') and 'int' [-Wsign-compare]
    9 |     FD_SET(fd, &fds);
      |     ^~~~~~~~~~~~~~~~
/usr/x86_64-w64-mingw32/sys-root/mingw/include/psdk_inc/_fd_types.h:77:40: note: expanded from macro 'FD_SET'
   77 |                 if (((fd_set *)(set))->fd_array[__i] == (fd)) {         \
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^   ~~

gcc gladly respects that _fd_types.h is a system header and is silent.

[CendioOssman]

Possible duplicate of #154015, but it's for a different component so I'm leaving this open for now.

[Eveeifyeve]

Would having the ability to control this ~~

llvm-project/clang/lib/Basic/Warnings.cpp

Lines 109 to 115 in 34fda63

	// -Wsystem-headers is a special case, not driven by the option table. It
	// cannot be controlled with -Werror.
	if (Opt == "system-headers") {
	if (SetDiagnostic)
	Diags.setSuppressSystemWarnings(!isPositive);
	continue;
	}

with -Werrors wouldn't it solve this issue?

UPDATE: indeed it does still not surpress, Linked duplicated to #154015