c++: For reflection look through DECL_LOCAL_DECL_P decls and fix up has_default_argum...

c++: For reflection look through DECL_LOCAL_DECL_P decls and fix up has_default_argument [PR123612]

Based on discussions in the PR that C++ 26 reflection is
entity based rather than declaration based, the following patch
ensures we reflect DECL_LOCAL_DECL_ALIAS of DECL_LOCAL_DECL_P vars
or function decls.
Additionally, while default arguments are intentionally not merged
from the block scope externs to their corresponding namespace scope
function decls, for std::meta::has_default_argument the wording
requires to return true even if there is at least one block scope
extern with default argument for the particular parameter.
So, the patch also records in a new flag whether a default argument
has been present in a block scope extern and propagates it through
further duplicate_decls.  eval_has_default_arguments then uses
both this new flag (for the block scope externs) and the preexisting
search for default arguments in the corresponding type.

2026-02-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123612
	* cp-tree.h (DECL_HAS_DEFAULT_ARGUMENT_P): Define.
	* decl.cc (merge_decl_arguments): For -freflection and
	extern_alias set DECL_HAS_DEFAULT_ARGUMENT_P (oldarg)
	if newdecl has a default argument for that parameter,
	otherwise propagate the flag.
	* reflect.cc (get_reflection): For DECL_LOCAL_DECL_P
	use its DECL_LOCAL_DECL_ALIAS.
	(eval_has_default_argument): Return boolean_true_node
	also if DECL_HAS_DEFAULT_ARGUMENT_P flag is set.

	* g++.dg/reflect/pr123612.C: New test.