[clang] Remove xbegin and _xend (PR #126952)

[Reid Kleckner via cfe-commits]

rnk wrote:

This is an observable behavior change, so it should have a test. There's a test for this header at clang/test/Headers/ms-intrins.cpp that you can extend. You probably need to add a new RUN line that adds `-mrtm` or some other micro-architectural feature so that rtmintrin.h declares `_xend` and you observe the redeclaration error reported in #95133.

https://github.com/llvm/llvm-project/pull/126952