demangler: Structured Bindings

demangler: Structured Bindings

C++ Structured bindings have a mangling that has yet to be formally
documented.  However, it's been around for a while and shows up for
module support.

	include/
	* demangle.h (enum demangle_component_type): Add
	DEMANGLE_COMPONENT_STRUCTURED_BINDING.
	libiberty/
	* cp-demangle.c (d_make_comp): Adjust.
	(d_unqualified_name): Add 'DC' support.
	(d_count_template_scopes): Adjust.
	(d_print_comp_inner): Add structured binding.
	* testsuite/demangle-expected: Add testcases.