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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> this change "likely" triggered build failures in https://lab.llvm.org/buildbot/#/builders/134/builds/33270. other culprit is #176885.
> 
> it seems like the tsan instrumented binaries just segfault after one of these changes. cmake config used by the buildbot is:
> 
> ```
> cmake -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DLLVM_USE_SANITIZER=Thread -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_OPTIMIZED_TABLEGEN=ON '-DLLVM_ENABLE_PROJECTS=llvm;clang-tools-extra;clang' '-DLLVM_LIT_ARGS=-v -vv' -GNinja ../llvm-project/llvm
> ```
> 
> relevant test target is `check-clangd`.
> 
> Can you verify if the breakage is triggered by this PR and revert/fix forward if so?

It's awfully odd that only clangd tests are failing for a change which impacts every test that involves lexing (which is... most Clang tests). This could be machine-specific given that it's using target attributes, unless there's something special about how tsan interacts with target attributes or something?

@Thibault-Monnier if there's not an obvious fix, I think it's best to revert while investigating what's going on. If you cannot reproduce the issue locally, we'll need some help from @kadircet or others impacted to help narrow down the parameters which cause the issue.

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