inhereit LLVM_ENABLE_LIBXML2

Summary: When building in an LLVM context, we should respect its LLVM_ENABLE_LIBXML2 option.

Reviewers: vitalybuka, mspertus, modocache

Reviewed By: modocache

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D53212

llvm-svn: 347870