c++: find_template_parameters and TEMPLATE_DECLs [PR101247]

c++: find_template_parameters and TEMPLATE_DECLs [PR101247]

r12-1989 fixed the testcase in the PR, but unfortunately the fix is
buggy: it breaks the case where the common template between the
TEMPLATE_DECL t and ctx_parms is the innermost template (as in
concepts-memtmpl5.C below).  This can be fixed by instead passing the
TREE_TYPE of ctmpl to common_enclosing_class when ctmpl is a class
template.

But even after that's fixed, the analogous case where the innermost
template is a partial specialization is still broken (as in
concepts-memtmpl5a.C below), because ctmpl is always a primary template.

So this patch instead takes a diferent approach that doesn't rely on
ctx_parms at all: when looking for the template parameters of a
TEMPLATE_DECL that are shared with the current template context, just
walk its DECL_CONTEXT.  As long as the template is not overly general
(e.g. we didn't pass it through most_general_template), this should give
us exactly what we want, since if a TEMPLATE_DECL can be referred to
from some template context then the template parameters it uses must all
be in-scope and contained in its DECL_CONTEXT.  This effectively makes
us treat TEMPLATE_DECLs more similarly to other _DECLs (whose DECL_CONTEXT
we also walk).

	PR c++/101247

gcc/cp/ChangeLog:

	* pt.c (any_template_parm_r) <case TEMPLATE_DECL>: Just walk the
	DECL_CONTEXT.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-memtmpl4.C: Uncomment the commented out
	example, which we now handle correctly.
	* g++.dg/cpp2a/concepts-memtmpl5.C: New test.
	* g++.dg/cpp2a/concepts-memtmpl5a.C: New test.