[ThinLTO] Fix lazy-loading of MDString instruction attachments

CFI is using intrinsics that takes MDString as arguments, and this
was broken during lazy-loading of metadata.

Differential Revision: https://reviews.llvm.org/D28916

llvm-svn: 292641

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -512,7 +512,7 @@ private:
   }
 
   Metadata *getFnMetadataByID(unsigned ID) {
-    return MDLoader->getMetadataFwdRef(ID);
+    return MDLoader->getMetadataFwdRefOrLoad(ID);
   }
 
   BasicBlock *getBasicBlock(unsigned ID) const {