[ASTImporter] Fix structural eq of lambdas

Summary:
The structural equivalence check reported false eq between lambda classes
with different parameters in their call signature.
The solution is to check the methods for equality too in case of lambda
classes.

Reviewers: a_sidorin, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64075

llvm-svn: 366332