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.

llvm-svn: 42818