[Concepts] Implement a portion of Concepts TS[dcl.spec.concept]p1 by

diagnosing when 'concept' is specified on a function or template
specialization.

Since a concept can only be applied to a function or variable template,
the concept bit is stored in TemplateDecl as a PointerIntPair.

Reviewers: rsmith, faisalv, aaron.ballman, hubert.reinterpretcast

Differential Revision: http://reviews.llvm.org/D13357

llvm-svn: 260074