c++: Tweaks for value_dependent_expression_p.

c++: Tweaks for value_dependent_expression_p.

We may not call value_dependent_expression_p on expressions that are
not potential constant expressions, otherwise value_d could crash,
as I saw recently (in C++98).  So beef up the checking in i_d_e_p.

This revealed a curious issue: when we have __PRETTY_FUNCTION__ in
a template function, we set its DECL_VALUE_EXPR to error_mark_node
(cp_make_fname_decl), so potential_c_e returns false when it gets it,
but value_dependent_expression_p handles it specially and says true.
This broke lambda-generic-pretty1.C.  So take care of that.

And then also tweak uses_template_parms.

gcc/cp/ChangeLog:

	* constexpr.c (potential_constant_expression_1): Treat
	__PRETTY_FUNCTION__ inside a template function as
	potentially-constant.
	* pt.c (uses_template_parms): Call
	instantiation_dependent_expression_p instead of
	value_dependent_expression_p.
	(instantiation_dependent_expression_p): Check
	potential_constant_expression before calling
	value_dependent_expression_p.