[ASTImporter] Regression test to ensure that we handling importing of anonymous enums correctly

Summary:
https://reviews.llvm.org/D51633 added error handling in the ASTImporter.cpp which uncovered an underlying bug in which we used the wrong name when handling naming conflicts. This could cause a segmentation fault when attempting to cast an int to an enum during expression parsing.

This test should pass once https://reviews.llvm.org/D59665 is committed.

Differential Revision: https://reviews.llvm.org/D59667

llvm-svn: 358462