Automerge: [libc++][ci] Update the Windows toolchains to Clang 19 (#129232)

This also fixes test failures in the clang-cl build configs that started
a couple days ago. It seems like the failures were triggered by an update
to the base image on the Github provided runners.

There were failures in test/libcxx/system_reserved_names.gen.py, due to
an issue in an Clang intrinsics header (avx512fp16intrin.h); this issue
was observed and fixed for Clang 19 in 6f04f46. The test does
    #define A SYSTEM_RESERVED_NAME
which clashes with a parameter with the name `A` in that header.

By upgrading the toolchain to Clang 19, we get fixed version of this
intrinsics header.

Also update the llvm-mingw toolchains to a version with Clang 19.1.7.

(cherry picked from commit e6a0ee3)

Author: mstorsjo