clang-cl: Add more tests for the interaction of /FI and /Yc /Yu.

clang-cl: Add more tests for the interaction of /FI and /Yc /Yu.

Most things even work; see the included FIXMEs for things that need polishing.

Also don't warn about unused flags for the `/Yuh2.h /FIh1.h /FIh2.h`.  The
common case is that the pch was built with `/Ych2.h /FIh1.h /FIh2.h`, so h1.h
is in the PCH, and we shouldn't warn about /FIh1.h not having an effect.
(If we wanted to get fancy, we could store the list of -include flags in the
pch and then check that it matches later on.)

llvm-svn: 264178