[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