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
diff --git a/llvm/lib/MC/MCSectionCOFF.cpp b/llvm/lib/MC/MCSectionCOFF.cpp
index ce0b4f5..b8373f4 100644
--- a/llvm/lib/MC/MCSectionCOFF.cpp
+++ b/llvm/lib/MC/MCSectionCOFF.cpp
@@ -11,6 +11,7 @@
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCSymbol.h"
+#include "llvm/Support/COFF.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;