[clang] [llvm] Reapply "[cmake] Add support for statically linking libxml2" (PR #192088)

[Keith Smiley via cfe-commits]

keith wrote:

the attempted windows fix here is small vs the original commit. im going to merge and watch the fuschia jobs here: https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-host-windows-x64

i will revert quickly if it breaks it again

https://github.com/llvm/llvm-project/pull/192088