Fix crash on missing header in -rewrite-includes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158459 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Rewrite/InclusionRewriter.cpp b/lib/Rewrite/InclusionRewriter.cpp
index 2dd6b34..3dfc3b0 100644
--- a/lib/Rewrite/InclusionRewriter.cpp
+++ b/lib/Rewrite/InclusionRewriter.cpp
@@ -250,7 +250,8 @@
 {
   bool Invalid;
   const MemoryBuffer &FromFile = *SM.getBuffer(FileId, &Invalid);
-  assert(!Invalid && "Invalid FileID while trying to rewrite includes");
+  if (Invalid) // invalid inclusion
+    return true;
   const char *FileName = FromFile.getBufferIdentifier();
   Lexer RawLex(FileId, &FromFile, PP.getSourceManager(), PP.getLangOpts());
   RawLex.SetCommentRetentionState(false);