[llvm] [cmake] Use the C++ linker when checking libxml2 (PR #221970)

[Hans Wennborg via llvm-commits]

https://github.com/zmodem approved this pull request.

This looks reasonable to me. Petr is the expert though, so let's see if he wants to add any comment as well.

https://github.com/llvm/llvm-project/pull/221970