[CMake] Simplify CMake handling for libxml2

[CMake] Simplify CMake handling for libxml2

This matches the changes made to handling of zlib done in 10b1b4a
where we rely on find_package and the imported target rather than
manually appending the library and include paths. The use of
LLVM_LIBXML2_ENABLED has been replaced by LLVM_ENABLE_LIBXML2
thus reducing the number of variables.

Differential Revision: https://reviews.llvm.org/D84563