Make a significant change to invert the control flow handling
predefined macros.  Previously, these were handled by the driver,
now they are handled by the preprocessor.

Some fallout of this:

1. Instead of preprocessing two buffers (the predefines, then the 
   main source file)  we now start preprocessing the main source 
   file and inject the predefines as a "psuedo #include" from the
   main source file.
2. #1 allows us to nuke the Lexer::IsMainFile flag and simplify
   Preprocessor::isInPrimaryFile.
3. The driver doesn't have to know about standard #defines, the
   preprocessor knows, which is nice for people wanting to define
   their own drivers.
4. This allows us to put normal tokens in the predefine buffer,
   for example a definition for __builtin_va_list that is 
   target-specific, and a typedef for id in objc.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@42818 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Driver/DiagChecker.cpp b/Driver/DiagChecker.cpp
index 3e276d7..4b45729 100644
--- a/Driver/DiagChecker.cpp
+++ b/Driver/DiagChecker.cpp
@@ -91,7 +91,7 @@
   PP.SetCommentRetentionState(true, true);
 
   // Enter the cave.
-  PP.EnterSourceFile(MainFileID, 0, true);
+  PP.EnterMainSourceFile(MainFileID);
 
   // Turn off all warnings from relexing or preprocessing.
   PP.getDiagnostics().setWarnOnExtensions(false);