c++: Handle TU_LOCAL_ENTITY in tsubst_expr and potential_constant_expression

c++: Handle TU_LOCAL_ENTITY in tsubst_expr and potential_constant_expression

This cleans up the TU_LOCAL_ENTITY handling to avoid unnecessary
tree walks and make the logic more robust.

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1): Handle
	TU_LOCAL_ENTITY.
	* pt.cc (expr_contains_tu_local_entity): Remove.
	(function_contains_tu_local_entity): Remove.
	(dependent_operand_p): Remove special handling for
	TU_LOCAL_ENTITY.
	(tsubst_expr): Handle TU_LOCAL_ENTITY when tsubsting OVERLOADs;
	remove now-unnecessary extra handling.
	(type_dependent_expression_p): Handle TU_LOCAL_ENTITY.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>