Windows: Disable clang headers when C-preprocessing with clang-cl (#4934)

As they apparently use unsupported types like __int128, __bf16 etc.,
even if preprocessing with `/std:c11`. Use the Microsoft headers
instead.

Author: kinke

driver/cpreprocessor.cpp

@@ -151,6 +151,10 @@ FileName runCPreprocessor(FileName csrcfile, Loc loc, OutBuffer &defines) {
 
       // need to redefine some macros in importc.h
       args.push_back("-Wno-builtin-macro-redefined");
+
+      // disable the clang resource headers (immintrin.h etc.), using
+      // unsupported types like __int128, __bf16 etc. - stick to the MS headers
+      args.push_back("-nobuiltininc");
     }
 
     args.push_back(csrcfile.toChars());

runtime/druntime/test/importc_compare/Makefile

@@ -21,10 +21,3 @@ endif
 include ../common.mak
 
 extra_dflags += -d
-
-ifeq ($(OS),windows)
-    ifdef IN_LDC
-        # use Microsoft's cl.exe as preprocessor; clang-cl leads to unsupported `_Float16` etc.
-        extra_dflags += -gcc=cl
-    endif
-endif

tests/dmd/compilable/stdcheaders.c

@@ -2,17 +2,6 @@
  * Many platforms do not support all the C Standard headers.
  */
 
-/* LDC defaults to clang-cl (if available) as preprocessor on Windows, which for
- * this test causes importC trouble starting with the MSVC v17.13 headers, wrt.
- * `__declspec(_Noreturn)`, `_Float16`, `__bf16`, `unsigned __int128` etc.
- *
- * => use Microsoft's cl.exe as preprocessor
- * REQUIRED_ARGS(windows): -gcc=cl
- *
- * => ignore cl.exe printing the preprocessed filename (auto-suppressed by DMD)
- * TRANSFORM_OUTPUT(windows): remove_lines("^stdcheaders\.c$")
- */
-
 #include <assert.h>
 
 #include <complex.h>