[ASTImporter] Fix structural eq of lambdas
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
Reviewers: a_sidorin, a.sidorin
Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits
Differential Revision: https://reviews.llvm.org/D64075