Add -cc1 -ast-dump-xml, an excessively detailed XML dump of the internals

Add -cc1 -ast-dump-xml, an excessively detailed XML dump of the internals
of the ASTs.  Only available in assertions builds.  No stability guarantee.

This is intended solely as a debugging tool.  I'm not sure if the goals
are sufficiently aligned with the XML printer to allow a common
implementation.

Currently just falls back on the StmtDumper to display statements,
which means it doesn't produce valid XML in those cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120088 91177308-0d34-0410-b5e6-96231b3b80d8