Put macro redefinition warnings under -Wmacro-redefined

This is consistent with -Wbuiltin-macro-redefined, and puts this common
extension warning under a flag.

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D3283

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205591 91177308-0d34-0410-b5e6-96231b3b80d8