[CONCEPTS] Parsing of concept keyword

Summary: This change adds parsing for the concept keyword in a
declaration and tracks the location. Diagnostic testing added for
invalid use of concept keyword.

Reviewers: faisalv, fraggamuffin, rsmith, hubert.reinterpretcast

Reviewed By: rsmith, hubert.reinterpretcast

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10528

Patch by Nathan Wilson!

llvm-svn: 241060