[clang-tidy] Optimize misc-confusable-identifiers

Main performance issue in this check were caused by many
calls to getPrimaryContext and constant walk up to declaration
contexts using getParent. Also there were issue with forallBases
that is slow.

Profiled with perf and tested on open-source project Cataclysm-DDA.
Before changes check took 27320 seconds, after changes 3682 seconds.
That's 86.5% reduction. More optimizations are still possible in this
check.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D151051