[libcxx] [ci] Update Clang on Windows (#152206)
Update clang-cl/LLVM to 20.1.8.
Update to llvm-mingw 20250709 (with also is built on LLVM 20.1.8). This
release of llvm-mingw is the first release to be built with PGO, making
it significantly faster for the CI runs (on par with the clang-cl
cases); running the current tests in around 1 h rather than 1 h 20 min.
