c++: Don't recurse on DECL_INITIAL for DECL_EXPR on non-VAR_DECLs [PR108606]

The r13-2965-g73d9b0e5947e16 change changed the line touched in this patch
from
      return RECUR (tmp, want_rval);
to
      return RECUR (DECL_INITIAL (tmp), want_rval);
This is on DECL_EXPR handling code, where tmp can be lots of different
trees and DECL_INITIAL unfortunately also means different things on
different trees.
It is the initializer on VAR_DECL, DECL_ARG_TYPE on PARM_DECLs (though
those are unlikely to have DECL_EXPRs), for FUNCTION_DECLs the body,
..., USING_DECL_DECLS on USING_DECLs and DECL_FRIENDLIST on TYPE_DECLs.

The testcase below ICEs because we have a DECL_EXPR for TYPE_DECL
which has non-NULL DECL_FRIENDLIST and we certainly can't recurse on
the friend list.

The following patch will RECUR on DECL_INITIAL only for VAR_DECLs and
for anything else just return true.

2023-03-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108606
	* constexpr.cc (potential_constant_expression_1) <case DECL_EXPR>:
	Only recurse on DECL_INITIAL (tmp) if tmp is a VAR_DECL, otherwise
	just return true.

	* g++.dg/cpp1y/pr108606.C: New test.