[ASTImporter][ASTImporterSpecificLookup] Add test for different operators

Summary:
This is to check that operators are handled properly in
`ASTImporterSpecificLookup`.  Note, this lookup table is not used in LLDB, only
in CTU.

Reviewers: a_sidorin, shafik, a.sidorin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57905

llvm-svn: 353505