[leaks] Parse the schema file every time we try to verify the XML. Yes,
this is wasteful, blah blah, but this is a test utility only. It turns
out that without doing this, libxml2 will always leak a bunch of the XML
data, and that is causing failures with LSan. This is also quite a bit
simpler and I don't think it is slow enough to really be a show stopper.
If someone yells about the runtime of c-index-test, we can do other
things to try to mitigate it, but the current strategy wasn't working
well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207882 91177308-0d34-0410-b5e6-96231b3b80d8