[CMake] Consistently use the LibXml2::LibXml2 target instead of LIBXML2_LIBRARIES

Linking against the LibXml2::LibXml2 target has the advantage of not only importing the library, but also adding the include path as well as any definitions the library requires. In case of a static build of libxml2, eg. a define is set on Windows to remove any DLL imports and export.

LLVM already makes use of the target, but c-index-test and lldb were still linking against the library only.

The workaround for Mac OS-X that I removed seems to have also been made redundant since https://reviews.llvm.org/D84563 I believe

Differential Revision: https://reviews.llvm.org/D109975