[Concepts] Parsing of requires-clause in template-declaration

Summary:
This change implements parse-only acceptance of the optional
requires-clause in a template-declaration. Diagnostic testing is added
for cases where the grammar is ambiguous with the expectation that the
longest token sequence which matches the syntax of a
constraint-expression is consumed without backtracking.

Reviewers: faisalv, fraggamuffin, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10462

llvm-svn: 240611