79316 observed that we were rejecting default arguments in a deduction guide, which is required by some of the usage of deduction guides in the standard library.
79350 observed that we weren't allowing explicit on a deduction guide, which was added at the last meeting. This patch fixes these issues; the first by parsing deduction guides more like we do constructors, so we treat them as a normal declarator rather than an abstract one (which doesn't allow default arguments). For the second, we needed to tell class deduction whether we're in copy- or direct-initialization context. Tested x86_64-pc-linux-gnu, applying to trunk.
