Debug Info: Use the full module name as a key when caching DIModules.

The signature may not have been computed at the time the module reference
is generated (e.g.: in the future while emitting debug info for a clang
module). Using the full module name is safe because each clang module may
only have a single definition.

NFC.

llvm-svn: 248037

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -1674,7 +1674,7 @@ llvm::DIType *CGDebugInfo::CreateType(const ObjCInterfaceType *Ty,
 
 llvm::DIModule *
 CGDebugInfo::getOrCreateModuleRef(ExternalASTSource::ASTSourceDescriptor Mod) {
-  auto &ModRef = ModuleRefCache[Mod.Signature];
+  auto &ModRef = ModuleRefCache[Mod.ModuleName];
   if (ModRef)
     return cast<llvm::DIModule>(ModRef);