c++: Fix folding of non-dependent BASELINKs [PR95468]

c++: Fix folding of non-dependent BASELINKs [PR95468]

Here, the problem ultimately seems to be that tsubst_copy_and_build,
when called with empty args as we do during non-dependent expression
folding, doesn't touch BASELINKs at all: it delegates to tsubst_copy
which then immediately exits early due to the empty args.  This means
that the CAST_EXPR int(1) in the BASELINK A::condition<int(1)> never
gets folded (as part of folding of the overall CALL_EXPR), which later
causes us to crash when performing overload resolution of the rebuilt
CALL_EXPR (which is still in terms of this templated BASELINK).

This doesn't happen when condition() is a namespace-scope function
because then condition<int(1)> is represented by a TEMPLATE_ID_EXPR
rather than by a BASELINK, which does get handled directly from
tsubst_copy_and_build.

This patch fixes this issue by having tsubst_copy_and_build handle
BASELINK directly rather than delegating to tsubst_copy, so that it
processes BASELINKs even when args is empty.

gcc/cp/ChangeLog:

	PR c++/95468
	* pt.c (tsubst_copy_and_build) <case BASELINK>: New case, copied
	over from tsubst_copy.

gcc/testsuite/ChangeLog:

	PR c++/95468
	* g++.dg/template/non-dependent15.C: New test.