[libcxx] [test] Don't pass INCLUDE to clang via -isystem

[mstorsjo]

Summary

Passing the MSVC include dirs via -isystem makes them included before clang's own include resource dir (<prefix>/lib/clang/<version>/include). This causes includes of stddef.h to bypass clang's stddef.h which defines max_align_t, which libc++ needs defined.

[mstorsjo]

To follow up on the commit description: Manually passing these via -isystem isn't needed as the compiler driver itself picks up the include directories (and places them at the correct place in the include order!) from the INCLUDE variable.

[mstorsjo]

Fwiw, I traced this code back to rG4372f06d0fcaba1a6913e2f37be064e06a7b5b5b where the initial windows testing support was added - unsure if it really was needed back then; right now it isn't - and since libc++ started depending on max_align_t, it became an issue.

libcxx/utils/libcxx/test/config.py

def _configure_clang_cl(self, clang_path):
             return [p.strip() for p in os.environ.get(var, '').split(';') if p.strip()]
 
         def _prefixed_env_list(var, prefix):
             from itertools import chain
             return list(chain.from_iterable((prefix, path) for path in _split_env_var(var)))
 
         assert self.cxx_is_clang_cl
         flags = []
-        compile_flags = _prefixed_env_list('INCLUDE', '-isystem')
+        compile_flags = []
         link_flags = _prefixed_env_list('LIB', '-L')
         for path in _split_env_var('LIB'):
             self.add_path(self.exec_env, path)
         return CXXCompiler(self, clang_path, flags=flags,
                            compile_flags=compile_flags,
                            link_flags=link_flags)
 
     def configure_src_root(self):