clang: Add a cmake toggle to disable libclang's libxml2 dependency

By default, libclang will still depend on libxml2 according to
LLVM_ENABLE_LIBXML2, but with this it's possible to make libclang
not depend on libxml2 even though LLVM does.

The motivation is that libclang uses a bigger API surface of libxml2
than LLVM, and we're statically linking a very minimal build of
libxml2 in our LLVM/clang distribution.

Differential Revision: https://reviews.llvm.org/D136700