[clang] [llvm] [Clang][Lexer] Reland "Detect SSE4.2 availability at runtime in fastParseASCIIIdentifier" (PR #175452)

[kadir çetinkaya via cfe-commits]

kadircet wrote:

> It's awfully odd that only clangd tests are failing

FWIW, we only build & test clangd (and other binaries from that subtree) on that buildbot

https://github.com/llvm/llvm-project/pull/175452