[clang][lex] Refactor check for the first file include

[clang][lex] Refactor check for the first file include

This patch refactors the code that checks whether a file has just been included for the first time.

The `HeaderSearch::FirstTimeLexingFile` function is removed and the information is threaded to the original call site from `HeaderSearch::ShouldEnterIncludeFile`. This will make it possible to avoid tracking the number of includes in a follow up patch.

Depends on D114092.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D114093