c++: Return only in-scope tparms in keep_template_parm [PR95310]

In the testcase below, the dependent specializations iter_reference_t<F>
and iter_reference_t<Out> share the same tree due to specialization
caching.  So when find_template_parameters walks through the
requires-expression (as part of normalization), it sees and includes the
out-of-scope template parameter F in the list of template parameters
it found within the requires-expression (along with Out and N).

From a correctness perspective this is harmless since the parameter mapping
routines only care about the level and index of each parameter, so F is
no different from Out in that sense.  And it's also harmless that two
parameters in the parameter mapping have the same level and index.

But having both Out and F in the parameter mapping means extra work for
hash_atomic_constrant, tsubst_parameter_mapping and get_mapped_args; and
it also means we print this irrelevant template parameter in the
testcase's diagnostics (via pp_cxx_parameter_mapping):

  in requirements with ‘Out o’ [with N = (const int&)&a; F = const int*; Out = const int*]

This patch makes keep_template_parm return only in-scope template
parameters by looking into ctx_parms for the corresponding in-scope
one, through a new helper function corresponding_template_parameter.

(That we sometimes print irrelevant template parameters in diagnostics
is also the subject of PR99 and PR66968, so the above diagnostic issue
could likely be fixed in a more general way, but this targeted fix to
keep_template_parm is perhaps worthwhile on its own.)

gcc/cp/ChangeLog:

	PR c++/95310
	* pt.c (corresponding_template_parameter): Define.
	(keep_template_parm): Use it to adjust the given template
	parameter to the corresponding in-scope one from ctx_parms.

gcc/testsuite/ChangeLog:

	PR c++/95310
	* g++.dg/concepts/diagnostic15.C: New test.

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