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

[Timm Baeder via cfe-commits]

tbaederr wrote:

> Do you have benchmarks? I wonder if we should use a small struct instead of pair, it would be slightly easier to maintain I think. Looks good otherwise.

See the commit: https://github.com/llvm/llvm-project/pull/70543/commits/c9d34dae319de3eed1a23c23ff7b6d7673a04b79

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