[cmake] Include LLVM_LIBXML2_ENABLED in LLVMConfig.cmake, PR36006

Include the LLVM_LIBXML2_ENABLED cache variable in LLVMConfig.cmake
in order to make it available for other LLVM packages to query. This
is necessary to fix stand-alone testing of LLD.

Differential Revision: https://reviews.llvm.org/D42252

llvm-svn: 322973