libiberty: Invoke D demangler when --format=auto

Investigating GDB PR d/31580 showed that the libiberty demangler
doesn't automatically demangle D mangled names.  However, I think it
should -- like C++ and Rust (new-style), D mangled names are readily
distinguished by the leading "_D", and so the likelihood of confusion
is low.  The other non-"auto" cases in this code are Ada (where the
encoded form could more easily be confused by ordinary programs) and
Java (which is long gone, but which also shared the C++ mangling and
thus was just an output style preference).

This patch also fixed another GDB bug, though of course that part
won't apply to the GCC repository.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31580
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30276

libiberty
	* cplus-dem.c (cplus_demangle): Try the D demangler with
	"auto" format.
	* testsuite/d-demangle-expected: Add --format=auto test.