c++: Implement DR 2289, Uniqueness of structured binding names [PR94553]

c++: Implement DR 2289, Uniqueness of structured binding names [PR94553]

DR 2289 clarified that since structured bindings have no C compatibility
implications, they should be unique in their declarative region, see
[basic.scope.declarative]/4.2.

The duplicate_decls hunk is the gist of the patch, but that alone would
not be enough to detect the 'A' case: cp_parser_decomposition_declaration
uses

13968       tree decl2 = start_decl (declarator, &decl_specs, SD_INITIALIZED,
13969                                NULL_TREE, NULL_TREE, &elt_pushed_scope);

to create the 'A' VAR_DECL but in this start_decl's grokdeclarator we
don't do fit_decomposition_lang_decl because the declarator kind is not
cdk_decomp, so then when start_decl calls maybe_push_decl, the decl 'A'
isn't DECL_DECOMPOSITION_P and we don't detect this case.  So I needed a
way to signal to start_decl that it should fit_decomposition_lang_decl.
In this patch, I'm adding SD_DECOMPOSITION flag to say that the variable
is initialized and it should also be marked as DECL_DECOMPOSITION_P.

	DR 2289
	PR c++/94553
	* cp-tree.h (SD_DECOMPOSITION): New flag.
	* decl.c (duplicate_decls): Make sure a structured binding is unique
	in its declarative region.
	(start_decl): If INITIALIZED is SD_DECOMPOSITION, call
	fit_decomposition_lang_decl.
	(grokdeclarator): Compare INITIALIZED directly to SD_* flags.
	* parser.c (cp_parser_decomposition_declaration): Pass SD_DECOMPOSITION
	to start_decl.

	* g++.dg/cpp1z/decomp52.C: New test.