[Concepts] Add diagnostic: non template declaration

Summary:
Adding diagnostic for concepts declared as non template (function
or variable)

Reviewers: faisalv, fraggamuffin, rsmith, hubert.reinterpretcast

Subscribers: nwilson, cfe-commits

Differential Revision: http://reviews.llvm.org/D11490

Patch by Nathan Wilson!

llvm-svn: 243690