Add PPCallbacks for #if/#ifdef/etc.

The callback info for #if/#elif is not great -- ideally it would give
us a list of tokens in the #if, or even better, a little parse tree.
But that's a lot more work.  Instead, clients can retokenize using
Lexer::LexFromRawLexer().

Reviewed by nlewycky.

llvm-svn: 118318