Move manifest utils into separate lib, to reduce libxml2 deps.

Summary:
Previously were in support.  Since many many things depend on support,
were all forced to also depend on libxml2, which we only want in a few cases.
This puts all the libxml2 deps in a separate lib to be used only in a few
places.

Reviewers: ruiu, thakis, rnk

Subscribers: mgorny, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D35819

llvm-svn: 309070