[SourceManager] Improve getFileIDLocal.

Prune the search space -- If we know offset(LastFileIDLookup) < SearchOffset, we
can prune the initial binary-search range from [0, end) to [LastFileIDlookup, end).

It reduces the binary search scan by ~30%.

SemaExpr.cpp:   1393437 -> 1035426
FindTarget.cpp: 1275930 -> 920087

Linux kernel:
getFileIDLocal: 2.45% -> 2.15%

Differential Revision: https://reviews.llvm.org/D135132