[clang][lex] Include tracking: simplify and move to preprocessor

This patch replaces the exact include count of each file in `HeaderFileInfo` with a set of included files in `Preprocessor`.

The number of includes isn't a property of a header file but rather a preprocessor state. The exact number of includes is not used anywhere except statistic tracking.

Reviewed By: vsapsai

Differential Revision: https://reviews.llvm.org/D114095