c++: Fix convert_like in template [PR91465, PR93870, PR92031, PR94068]

c++: Fix convert_like in template [PR91465, PR93870, PR92031, PR94068]

The point of this patch is to fix the recurring problem of trees
generated by convert_like while processing a template that break when
substituting.  For instance, when convert_like creates a CALL_EXPR
while in a template, substituting such a call breaks in finish_call_expr
because we have two 'this' arguments.  Another problem is that we
can create &TARGET_EXPR<> and then fail when substituting because we're
taking the address of an rvalue.  I've analyzed some of the already fixed
PRs and also some of the currently open ones:

In c++/93870 we create EnumWrapper<E>::operator E(&operator~(E)).
In c++/87145 we create S::operator int (&{N}).
In c++/92031 we create &TARGET_EXPR <0>.

The gist of the problem is when convert_like_real creates a call for
a ck_user or wraps a TARGET_EXPR in & in a template.  So in these cases
use IMPLICIT_CONV_EXPR.  In a template we shouldn't need to perform the
actual conversion, we only need it's result type.
perform_direct_initialization_if_possible and
perform_implicit_conversion_flags can also create an IMPLICIT_CONV_EXPR.

Given the change above, build_converted_constant_expr can return an
IMPLICIT_CONV_EXPR so call fold_non_dependent_expr rather than
maybe_constant_value to deal with that.

To avoid the problem of instantiating something twice in a row I'm
removing a call to instantiate_non_dependent_expr_sfinae in
compute_array_index_type_loc.  And the build_converted_constant_expr
pattern can now be simplified.

2020-03-09  Marek Polacek  <polacek@redhat.com>

	PR c++/92031 - bogus taking address of rvalue error.
	PR c++/91465 - ICE with template codes in check_narrowing.
	PR c++/93870 - wrong error when converting template non-type arg.
	PR c++/94068 - ICE with template codes in check_narrowing.
	* call.c (convert_like_real): Return IMPLICIT_CONV_EXPR
	in a template when not ck_identity and we're dealing with a class.
	(convert_like_real) <case ck_ref_bind>: Return IMPLICIT_CONV_EXPR
	in a template if we need a temporary.
	* decl.c (compute_array_index_type_loc): Remove
	instantiate_non_dependent_expr_sfinae call.  Call
	fold_non_dependent_expr instead of maybe_constant_value.
	(build_explicit_specifier): Don't instantiate or create a sentinel
	before converting the expression.
	* except.c (build_noexcept_spec): Likewise.
	* pt.c (convert_nontype_argument): Don't build IMPLICIT_CONV_EXPR.
	Set IMPLICIT_CONV_EXPR_NONTYPE_ARG if that's what
	build_converted_constant_expr returned.
	* typeck2.c (check_narrowing): Call fold_non_dependent_expr instead
	of maybe_constant_value.

	* g++.dg/cpp0x/conv-tmpl2.C: New test.
	* g++.dg/cpp0x/conv-tmpl3.C: New test.
	* g++.dg/cpp0x/conv-tmpl4.C: New test.
	* g++.dg/cpp0x/conv-tmpl5.C: New test.
	* g++.dg/cpp0x/conv-tmpl6.C: New test.
	* g++.dg/cpp1z/conv-tmpl1.C: New test.