[CONCEPTS] Add concept to VarDecl and diagnostic for uninitialized variable concept

Summary: Add IsConcept bit to VarDecl::NonParmVarDeclBitfields and associated isConcept/setConcept member functions. Set IsConcept to true when 'concept' specifier is in variable declaration. Create diagnostic when variable concept is not initialized.

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

Subscribers: aemerson, cfe-commits

Differential Revision: http://reviews.llvm.org/D11600

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243876 91177308-0d34-0410-b5e6-96231b3b80d8