[cmake] Include LLVM_LIBXML2_ENABLED in LLVMConfig.cmake, PR36006
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@322973 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm