c++: Implement DR 1512, Pointer comparison vs qual convs [PR87699]

c++: Implement DR 1512, Pointer comparison vs qual convs [PR87699]

This patch resolves DR 1512 (and, by turn, DR 583).  This entails:

1) Relational pointer comparisons against null pointer constants have
   been made ill-formed:

   void f(char *p) {
      if (p > 0)
	// ...
   }

   was always invalid in C but was -- accidentally -- allowed in C++.

2) This was ill-formed:

   bool foo(int** x, const int** y) {
     return x < y;
   }

   because 'int**' couldn't be converted to 'const int**'.  This was
   fixed by re-defining a generic composite pointer type.  The composite
   type of these two pointers will be 'const int *const *', to which
   both pointers can be converted.

3) The overload descriptions for built-in operators were adjusted,
   because objects of type std::nullptr_t cannot be used with relational
   operators any more.

I fixed 1) by adjusting cp_build_binary_op; we already had a warning
for it so made it a hard error now.

Then 2) required tweaking composite_pointer_type_r.  [expr.type] defines
the composite pointer type by using the "cv-combined type."  We didn't
implement the [conv.qual]/3.3 part; previously the composite type of
'int**' and 'const int**' was 'const int**', so this didn't compile:

    void f(const int **p, int **q) {
      true ? p : q;
    }

I wrote a more extensive test for this which uses decltype and some
template magic to check the composite type, see composite-ptr-type.C.
We still don't handle everything that [expr.type] requires us to,
but it's pretty close.

And finally 3) was handled in add_builtin_candidate.  Turned out we
weren't creating built-in operator candidates when the type was
std::nullptr_t at all.  We should, for == and !=.  Tested in builtin4.C.
In passing, I'm fixing some of the comments too.

	DR 1512
	PR c++/87699
	* call.c (add_builtin_candidate) <case EQ_EXPR>: Create candidate
	operator functions when type is std::nullptr_t for ==/!=.
	* typeck.c (composite_pointer_type_r): Add bool a * parameter.  Use it
	to maybe add "const" to the pointer type.
	(composite_pointer_type): Update the call to composite_pointer_type_r.
	(cp_build_binary_op): Turn two warning_at into error_at.  Print the
	types.

	* g++.dg/cpp0x/constexpr-array-ptr10.C: Change dg-warning to dg-error
	and adjust the expected messages in dg-error.
	* g++.dg/expr/composite-ptr-type.C: New test.
	* g++.dg/expr/ptr-comp1.C: New test.
	* g++.dg/expr/ptr-comp2.C: New test.
	* g++.dg/expr/ptr-comp3.C: New test.
	* g++.dg/overload/builtin4.C: New test.
	* g++.dg/warn/Wextra-3.C: Change dg-warning to dg-error.