Add SBDebugger::GetBuildConfiguration and use it to skip an XML test

Summary:
This adds a SBDebugger::GetBuildConfiguration static function, which
returns a SBStructuredData describing the the build parameters of
liblldb. Right now, it just contains one entry: whether we were built
with XML support.

I use the new functionality to skip a test which requires XML support,
but concievably the new function could be useful to other liblldb
clients as well (making sure the library supports the feature they are
about to use).

Reviewers: zturner, jingham, clayborg, davide

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D43333

llvm-svn: 325504