[clang][CrossTU] Invalidate parent map after get cross TU definition.

Summary:
Parent map of ASTContext is built once. If this happens and later
the TU is modified by getCrossTUDefinition the parent map does not
contain the newly imported objects and has to be re-created.

Invalidation of the parent map is added to the CrossTranslationUnitContext.
It could be added to ASTImporter as well but for now this task remains the
responsibility of the user of ASTImporter. Reason for this is mostly that
ASTImporter calls itself recursively.

Reviewers: gamesh411, martong

Reviewed By: gamesh411

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82568