[clang:sema] de-duplicate getDepthAndIndex helpers

Summary:
Continuing off of:
https://reviews.llvm.org/D38382

Fixes:
https://bugs.llvm.org/show_bug.cgi?id=12176

Reviewers: srhines, pirama, vsk

Reviewed By: vsk

Subscribers: cfe-commits, vsk, maitesin

Differential Revision: https://reviews.llvm.org/D49760

llvm-svn: 337944