[clang] Track how headers get included generally during lookup time

tapi & clang-extractapi both attempt to construct then check against
how a header was included to determine api information when working
against multiple search paths, headermap, and vfsoverlay mechanisms.
Validating this against what the preprocessor sees during lookup time
makes this check more reliable.

Reviewed By: zixuw, jansvoboda11

Differential Revision: https://reviews.llvm.org/D124638