Entering the main source file in the preprocessor can fail if the
source file has been changed. Handle that failure more gracefully.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98727 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/CacheTokens.cpp b/lib/Frontend/CacheTokens.cpp
index 02d6cec..199be3d 100644
--- a/lib/Frontend/CacheTokens.cpp
+++ b/lib/Frontend/CacheTokens.cpp
@@ -549,7 +549,8 @@
   // Lex through the entire file.  This will populate SourceManager with
   // all of the header information.
   Token Tok;
-  PP.EnterMainSourceFile();
+  if (PP.EnterMainSourceFile())
+    return;
   do { PP.Lex(Tok); } while (Tok.isNot(tok::eof));
 
   // Generate the PTH file.