Handle PCHReader::IgnorePCH, it gets returned for when the file does not exist.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74196 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/ASTUnit.cpp b/lib/Frontend/ASTUnit.cpp
index a7fbcda..8143263 100644
--- a/lib/Frontend/ASTUnit.cpp
+++ b/lib/Frontend/ASTUnit.cpp
@@ -113,14 +113,10 @@
     break;
     
   case PCHReader::Failure:
-    // Unrecoverable failure: don't even try to process the input
-    // file.
+  case PCHReader::IgnorePCH:
     if (ErrMsg)
       *ErrMsg = "Could not load PCH file";
     return NULL;
-
-  case PCHReader::IgnorePCH:
-    assert(0 && "Is there a validation that should not have happened ?");
   }
   
   // PCH loaded successfully. Now create the preprocessor.