Use isFromMainFile instead of comparing FileIDs directly.

llvm-svn: 49687
diff --git a/clang/Driver/RewriteTest.cpp b/clang/Driver/RewriteTest.cpp
index 55c5f1a..1c54c5a 100644
--- a/clang/Driver/RewriteTest.cpp
+++ b/clang/Driver/RewriteTest.cpp
@@ -384,7 +384,7 @@
     RewriteForwardProtocolDecl(FP);
   }
   // If we have a decl in the main file, see if we should rewrite it.
-  if (SM->getDecomposedFileLoc(Loc).first == MainFileID)
+  if (SM->isFromMainFile(Loc))
     return HandleDeclInMainFile(D);
 }