Mimicing MSCV, clang-cl doesn't predefine __STDC__
(InitializeStandardPredefinedMacros in clang/lib/Frontend/InitPreprocessor.cpp)
when LangOpts.MSVCCompat aka -fms-compatibility is set, which it is by default
for clang-cl (Clang::ConstructJob in clang/lib/Driver/ToolChains/Clang.cpp).

But clang-cl still needs to be called with /FIIntrin.h to preload declarations
of intrinsics from the Clang-provided Intrin.h, some of which like

  unsigned char _bittest(long const *, long);

both make use of "const" (which isn't defined away because the file is loaded so
early) and are duplicated in the system-provided winnt.h---where the "const"
/is/ defined away when that header is included later, causing "conflicting
types" errors.

("Real" MSVC also runs into the #if defining "const" away, but it seems it
happens to not have bad consequences there.)

Change-Id: Ice92a059330bd945f8a3d9e5fa1635a01d08ad8b
Reviewed-on: https://gerrit.libreoffice.org/60321
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>