Split out header integration tests

These are somewhat arbitrary tests that check if "thing goes fine" when
processing various platform-specific headers.

Also move warn-sysheader.cpp to Misc where the other diagnostics infrastructure
tests live.

File moves only.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207936 91177308-0d34-0410-b5e6-96231b3b80d8