PPCallbacks::MacroUndefined, change signature and add test.

Summary:
The PPCallbacks::MacroUndefined callback is currently insufficient for clients that need to track the MacroDirectives.
This patch adds an additional argument to PPCallbacks::MacroUndefined that is the undef MacroDirective.

Reviewers: bruno, manmanren

Reviewed By: bruno

Subscribers: nemanjai, cfe-commits

Differential Revision: https://reviews.llvm.org/D29923

llvm-svn: 301449