[AST] Check described template at structural equivalence check.

Summary:
When checking a class or function the described class or function template
is checked too.
Split StructuralEquivalenceContext::Finish into multiple functions.
Improved test with symmetric check, added new tests.

Reviewers: martong, a.sidorin, a_sidorin, bruno

Reviewed By: martong, a.sidorin

Subscribers: rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D49223

llvm-svn: 339256