[clang] extend getCommonSugaredType to merge sugar nodes

[clang] extend getCommonSugaredType to merge sugar nodes

This continues D111283 by extending the getCommonSugaredType
implementation to also merge non-canonical type nodes.

We merge these nodes by going up starting from the canonical
node, calculating their merged properties on the way.

If we reach a pair that is too different, or which we could not
otherwise unify, we bail out and don't try to keep going on to
the next pair, in effect striping out all the remaining top-level
sugar nodes. This avoids mismatching 'companion' nodes, such as
ElaboratedType, so that they don't end up elaborating some other
unrelated thing.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>
Differential Revision: https://reviews.llvm.org/D130308