[libc++] Disable pragma system_header in the new testing configuration

[ldionne]

Summary

The new testing configuration did not turn off #pragma system_header,
which means we were not seeing warnings in system headers.

I expect this to start failing on GCC (see the removed comment), but
I'd like to fix the root cause before I ship this.

[ldionne]

Fix a bunch of warnings - I expect there are still some issues.

[ldionne]

Don't enable warnings in system headers for GCC - there's too many to fix for now.

libcxx/utils/libcxx/test/config.py

def configure_default_compile_flags(self):
         # being elided.
         if self.target_info.is_windows() and self.debug_build:
             self.cxx.compile_flags += ['-D_DEBUG']
 
         # Add includes for support headers used in the tests.
         support_path = os.path.join(self.libcxx_src_root, 'test/support')
         self.cxx.compile_flags += ['-I' + support_path]
 
-        # On GCC, the libc++ headers cause errors due to throw() decorators
-        # on operator new clashing with those from the test suite, so we
-        # don't enable warnings in system headers on GCC.
-        if self.cxx.type != 'gcc':
-            self.cxx.compile_flags += ['-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER']
-
         # Add includes for the PSTL headers
         pstl_src_root = self.get_lit_conf('pstl_src_root')
         pstl_obj_root = self.get_lit_conf('pstl_obj_root')
         if pstl_src_root is not None and pstl_obj_root is not None:
             self.cxx.compile_flags += ['-I' + os.path.join(pstl_src_root, 'include')]
             self.cxx.compile_flags += ['-I' + os.path.join(pstl_obj_root, 'generated_headers')]
             self.cxx.compile_flags += ['-I' + os.path.join(pstl_src_root, 'test')]
             self.config.available_features.add('parallel-algorithms')

libcxx/utils/libcxx/test/features.py

Feature(name='is-lockfree-runtime-function',
             int main(int, char**) { return x.is_lock_free(); }
           """)),
 
   Feature(name='apple-clang',                                                                                                      when=_isAppleClang),
   Feature(name=lambda cfg: 'apple-clang-{__clang_major__}'.format(**compilerMacros(cfg)),                                          when=_isAppleClang),
   Feature(name=lambda cfg: 'apple-clang-{__clang_major__}.{__clang_minor__}'.format(**compilerMacros(cfg)),                        when=_isAppleClang),
   Feature(name=lambda cfg: 'apple-clang-{__clang_major__}.{__clang_minor__}.{__clang_patchlevel__}'.format(**compilerMacros(cfg)), when=_isAppleClang),
 
-  Feature(name='clang',                                                                                                            when=_isClang),
+  Feature(name='clang',                                                                                                            when=_isClang,
+          actions=[AddCompileFlag('-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER')]),
   Feature(name=lambda cfg: 'clang-{__clang_major__}'.format(**compilerMacros(cfg)),                                                when=_isClang),
   Feature(name=lambda cfg: 'clang-{__clang_major__}.{__clang_minor__}'.format(**compilerMacros(cfg)),                              when=_isClang),
   Feature(name=lambda cfg: 'clang-{__clang_major__}.{__clang_minor__}.{__clang_patchlevel__}'.format(**compilerMacros(cfg)),       when=_isClang),
 
   Feature(name='gcc',                                                                                                              when=_isGCC),
   Feature(name=lambda cfg: 'gcc-{__GNUC__}'.format(**compilerMacros(cfg)),                                                         when=_isGCC),
   Feature(name=lambda cfg: 'gcc-{__GNUC__}.{__GNUC_MINOR__}'.format(**compilerMacros(cfg)),                                        when=_isGCC),
   Feature(name=lambda cfg: 'gcc-{__GNUC__}.{__GNUC_MINOR__}.{__GNUC_PATCHLEVEL__}'.format(**compilerMacros(cfg)),                  when=_isGCC),