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

[Rainer Orth via cfe-commits]

rorth wrote:

This patch broke builders like [Solaris/amd64](https://lab.llvm.org/staging/#/builders/8/builds/761) that include `clang-tools-extra`.

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