[clang-tools-extra] reimplement PreprocessorTracker in terms of StringSet.

Summary:
PreprocessorTracker is the last user of the old StringPool class, which
isn't super loved and isn't a great improvement over a plan StringSet.
Once this goes in we can remove StringPool entirely.

This is as discussed on cfe-dev.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78273