Make the empty headers used by cl-pch-showincludes.cpp unique so that
filesystems that link these files together by contents will not see
different behaviour in this test, which is not testing linked files
specifically.
This was uncovered by 5ea78c4113f8 which made us stop mutating the name
of the presumed loc for the file in ContentCache, but that just surfaced
an underlying issue that the filename of multiple includes of linked
files are not separately tracked.
Differential Revision: https://reviews.llvm.org/D135373
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
