[clang] Improve getFileIDLocal binary search. (#146510)
Avoid reading the `LocalSLocEntryTable` twice per loop iteration. NFC.
https://llvm-compile-time-tracker.com/compare.php?from=0b6ddb02efdcbdac9426e8d857499ea0580303cd&to=1aa335ccfb07ba96177b89b1933aa6b980fa14f6&stat=instructions:u
