[clang] Perf/lexer faster slow get char and size (PR #70543)

[Nico Weber via cfe-commits]

nico wrote:

This breaks building: http://45.33.8.238/linux/121901/step_4.txt

Please take a look and revert for now if it takes a while to fix.

https://github.com/llvm/llvm-project/pull/70543