[libcxx] [test] Fix the include_as_c.sh.cpp test in MSVC configurations

[mstorsjo]

Summary

Avoid including a header that is known not to work with clang in MSVC
mode when compiling as C.

(Alternatively, this could be something like "XFAIL: clang && msvc",
but I think it's more useful to actually check the rest of the test
instead of expecting the whole test to fail.)

libcxx/test/libcxx/include_as_c.sh.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // We're building as C, so this test doesn't work when building with modules.
 // UNSUPPORTED: -fmodules
 
 // GCC complains about unrecognized arguments because we're compiling the
 // file as C, but we're passing C++ flags on the command-line.
 // UNSUPPORTED: gcc
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 // Test that the C wrapper headers can be included when compiling them as C.
 
 // NOTE: It's not common or recommended to have libc++ in the header search
 // path when compiling C files, but it does happen often enough.
 
 // RUN: %{cxx} -c -xc %s -fsyntax-only %{flags} %{compile_flags} -std=c99
 
 #include <__config>

 #include <math.h>
 #include <setjmp.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <tgmath.h>
+// The clang-shipped tgmath.h header doesn't work with MSVC/UCRT's complex
+// headers in C mode, see PR46207.
+#ifndef _MSC_VER
+#    include <tgmath.h>
+#endif
 #include <wchar.h>
 #include <wctype.h>
 
 int main(int argc, char **argv) {
   (void)argc;
   (void)argv;
   return 0;
 }