Serialization: fix previous changes for upstream update

We would previously get an error for an unexpected EOF.  However, we no
longer were checking that case, and would fall through to the end of the
function, without a return value.  Return the proper failure indicator.

lib/Serialization/ModuleFile.cpp

@@ -555,6 +555,8 @@ bool ModuleFile::readIndexBlock(llvm::BitstreamCursor &cursor) {
       break;
     }
   }
+
+  return false;
 }
 
 class ModuleFile::DeclCommentTableInfo {
@@ -689,6 +691,8 @@ bool ModuleFile::readCommentBlock(llvm::BitstreamCursor &cursor) {
       break;
     }
   }
+
+  return false;
 }
 
 static Optional<swift::LibraryKind> getActualLibraryKind(unsigned rawKind) {