Reenable FindLibXml2 on Windows and confirm its at least 2.8 or newer

Summary:
GnuWin32 installs libxml2 2.4. This isn't recent enough for lldb's
usage and thus the build fails. Searching for libxml2 was disabled
due to this build failure.

However, the gdb-remote plugin requires libxml2 to parse various
gdb-remote protocol packets. Thus check and confirm that the libxml2
version is at least 2.8 and disable it if not.

Reviewers: compnerd, zturner

Reviewed By: compnerd

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D55583

llvm-svn: 349538