Unit test for Symtab::InitNameIndexes

Summary: In order to exploit the potential of LLVM's new ItaniumPartialDemangler for indexing in LLDB, we expect conceptual changes in the implementation of the InitNameIndexes function. Here is a unit test that aims at covering all relevant code paths in that function.

Reviewers: labath, jingham, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: friss, teemperor, davide, clayborg, erik.pilkington, lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D49909

llvm-svn: 338695