dwarf2.c: mangle_style

non_mangled incorrectly returned "true" for Ada.  Correct that, and
add a few more non-mangled entries.  Return a value suitable for
passing to cplus_demangle to control demangling.

	* dwarf2.c: Include demangle.h.
	(mangle_style): Rename from non_mangled.  Return DMGL_* value
	to suit lang.  Adjust all callers.