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.

llvm-svn: 246784

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -32,6 +32,7 @@
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbolELF.h"
 #include "llvm/MC/MCValue.h"
+#include "llvm/Support/COFF.h"
 #include "llvm/Support/Dwarf.h"
 #include "llvm/Support/ELF.h"
 #include "llvm/Support/ErrorHandling.h"