Serialization: Stop reading the control block if the format version doesn't match

rdar://128551774

lib/Serialization/ModuleFileSharedCore.cpp

@@ -334,6 +334,12 @@ static ValidationInfo validateControlBlock(
         LLVM_FALLTHROUGH;
       case 3:
         result.shortVersion = blobData.slice(0, scratch[2]);
+
+        // If the format version doesn't match, give up after also getting the
+        // compiler version. This provides better diagnostics.
+        if (result.status != Status::Valid)
+          return result;
+
         LLVM_FALLTHROUGH;
       case 2:
       case 1: