c++: Set OLD_PARM_DECL_P even in regenerate_decl_from_template [PR124306]

The following testcase ICEs, because we try to instantiate the PARM_DECLs
of foo <int> twice, once when parsing ^^foo <int> and remember in a
REFLECT_EXPR a PARM_DECL in there, later on regenerate_decl_from_template
is called and creates new set of PARM_DECLs and changes DECL_ARGUMENTS
(or something later on in that chain) to the new set.
This means when we call parameters_of on ^^foo <int> later on, they won't
compare equal to the earlier acquired ones, and when we do e.g. type_of
or other operation on the old PARM_DECL where it needs to search the
DECL_ARGUMENTS (DECL_CONTEXT (parm_decl)) list, it will ICE because it
won't find it there.

The following patch fixes it similarly to how duplicate_decls deals
with those, by setting OLD_PARM_DECL_P flag on the old PARM_DECLs, so that
before using reflections of those we search DECL_ARGUMENTS and find the
corresponding new PARM_DECL.

2026-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/124306
	* pt.cc (regenerate_decl_from_template): Mark the old PARM_DECLs
	replaced with tsubst_decl result with OLD_PARM_DECL_P flag.

	* g++.dg/reflect/parameters_of8.C: New test.