Emit warning if define or undef reserved identifier or keyword.

Summary:
This change implements warnings if macro name is identical to a keyword or
reserved identifier. The warnings are different depending on the "danger"
of the operation. Defining macro that replaces a keyword is on by default.
Other cases produce warning that is off by default but can be turned on
using option -Wreserved-id-macro.

This change fixes PR11488.

Reviewers: rnk

Reviewed By: rnk

Subscribers: rnk, cfe-commits

Differential Revision: http://reviews.llvm.org/D6194