Sink COFF.h MC include into .cpp files

This prevents MC clients from getting COFF.h, which conflicts with
winnt.h macros. Also a minor IWYU cleanup. Now the only public headers
including COFF.h are in Object, and they actually need it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246784 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed