Mangling: use macros instead of hard-coded swift symbol names.

This makes it easier to switch between the old and new mangling scheme.

lib/IRGen/IRGenModule.cpp

@@ -19,6 +19,7 @@
 #include "swift/AST/DiagnosticsIRGen.h"
 #include "swift/AST/IRGenOptions.h"
 #include "swift/Basic/Dwarf.h"
+#include "swift/Basic/ManglingMacros.h"
 #include "swift/ClangImporter/ClangImporter.h"
 #include "swift/Runtime/RuntimeFnWrappersGen.h"
 #include "swift/Runtime/Config.h"
@@ -642,8 +643,9 @@ llvm::Constant *IRGenModule::getEmptyTupleMetadata() {
   if (EmptyTupleMetadata)
     return EmptyTupleMetadata;
 
-  EmptyTupleMetadata =
-      Module.getOrInsertGlobal("_TMT_", FullTypeMetadataStructTy);
+  EmptyTupleMetadata = Module.getOrInsertGlobal(
+                          MANGLE_AS_STRING(METADATA_SYM(EMPTY_TUPLE_MANGLING)),
+                          FullTypeMetadataStructTy);
   if (Triple.isOSBinFormatCOFF())
     cast<llvm::GlobalVariable>(EmptyTupleMetadata)
         ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);