[ASTImporter][Structural Eq] Check for isBeingDefined

Summary:
If one definition is currently being defined, we do not compare for
equality and we assume that the decls are equal.

Reviewers: a_sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, cfe-commits

Differential Revision: https://reviews.llvm.org/D53697

llvm-svn: 345760