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

[Keith Smiley via cfe-commits]

keith wrote:

in https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8684437193602933057/+/u/clang/configure/stdout it at least found libxml this time:

```
-- Looking for xmlReadMemory
-- Looking for xmlReadMemory - found
```

so i think we're good on the original issue (looks like main is broken for other reasons so that job might still fail)

let me know if you see other issues that might be related to this!

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