[AST] Structural equivalence of methods

Summary:
Added structural equivalence check for C++ methods.
Improved structural equivalence tests.
Added related ASTImporter tests.

Reviewers: a.sidorin, szepet, xazax.hun, martong, a_sidorin

Reviewed By: martong, a_sidorin

Subscribers: a_sidorin, rnkovacs, cfe-commits

Differential Revision: https://reviews.llvm.org/D48628

llvm-svn: 336776