[CONCEPTS] Add diagnostics: non-defining function; non-namespace scope

Summary:
Create diagnostic for function concept declaration which is not a
definition.

Create diagnostic for concept declaration which isn't in namespace
scope.

Create associated tests.

Reviewers: rsmith, faisalv, fraggamuffin, hubert.reinterpretcast

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11027

Patch by Nathan Wilson!

llvm-svn: 242899