Re-land "Fix Bug 30978 by emitting cv file checksums."

This reverts r313431 and brings back r313374 with a fix to write
checksums as binary data and not ASCII hex strings.

llvm-svn: 313657

llvm/lib/IR/DebugInfoMetadata.cpp

@@ -354,6 +354,8 @@ DISubroutineType *DISubroutineType::getImpl(LLVMContext &Context, DIFlags Flags,
   DEFINE_GETIMPL_STORE(DISubroutineType, (Flags, CC), Ops);
 }
 
+// FIXME: Implement this string-enum correspondence with a .def file and macros,
+// so that the association is explicit rather than implied.
 static const char *ChecksumKindName[DIFile::CSK_Last + 1] = {
   "CSK_None",
   "CSK_MD5",