[clang] Speedup LineOffsetMapping::get

LineOffsetMapping::get is a critical function that consistently appears
in the top 5 more computation intensive functions when running the
preprocessor.

This change brings consistent speedup of ~.5% on, preprocessing time,
see

https://llvm-compile-time-tracker.com/compare.php?from=0745b0c0354a0c8e1fefb68a3876d15db6c2e27a&to=460f3f04dac025e6952d78fce104a88151508a29&stat=instructions:u

for detailed statistics.

Differential Revision: https://reviews.llvm.org/D138474