libiberty: Update D symbol demangling for latest ABI spec.

Some small improvements and clarifications have been done in the D ABI
specification to remove all ambiguities found in the current grammar,
this implementation now more closely resembles the spec, whilst
maintaining compatibility with the old ABI.

Three new rules have been added to the ABI.

1. Back references using 'Q', analogous to C++ substitutions, compresses
   repeated identifiers, types, and template symbol and value parameters.

2. Template aliases to externally mangled symbols are prefixed with 'X'.
   This includes any symbol that isn't extern(D), or has its name
   overriden with pragma(mangle).  This fixes an ambiguity where it was
   not clear whether 'V' was an encoded calling convention, or the next
   template value parameter.

3. Alias parameters, templates, and tuple symbols no longer encode the
   symbol length of its subpart.  Tuples are now terminated with 'Z'.
   This fixes another ambiguity where the first character of the mangled
   name can be a digit as well, so the demangler had to figure out where
   to split the two adjacent numbers by trying out each combination.

libiberty/ChangeLog:

	* d-demangle.c (enum dlang_symbol_kinds): Remove enum.
	(struct dlang_info): New struct
	(dlang_decode_backref): New function.
	(dlang_backref): New function.
	(dlang_symbol_backref): New function.
	(dlang_type_backref): New function.
	(dlang_symbol_name_p): New function.
	(dlang_function_type_noreturn): New function.
	(dlang_function_type): Add 'info' parameter.  Decode function type
	with dlang_function_type_noreturn.
	(dlang_function_args): Add 'info' parameter.
	(dlang_type): Add 'info' parameter.  Handle back referenced types.
	(dlang_identifier): Replace 'kind' parameter with 'info'.  Handle back
	referenced symbols.  Split off decoding of plain identifiers to...
	(dlang_lname): ...here.
	(dlang_parse_mangle): Replace 'kind' parameter with 'info'.  Decode
	function type and return with dlang_type.
	(dlang_parse_qualified): Replace 'kind' parameter with 'info', add
	'suffix_modifier' parameter.  Decode function type with
	dlang_function_type_noreturn.
	(dlang_parse_tuple): Add 'info' parameter.
	(dlang_template_symbol_param): New function.
	(dlang_template_args): Add 'info' parameter.  Decode symbol parameter
	with dlang_template_symbol_param.  Handle back referenced values, and
	externally mangled parameters.
	(dlang_parse_template): Add 'info' parameter.
	(dlang_demangle_init_info): New function.
	(dlang_demangle): Initialize and pass 'info' parameter.
	* testsuite/d-demangle-expected: Add new tests.

Co-Authored-By: Rainer Schuetze <r.sagitario@gmx.de>