c: Implement C2y N3517 array subscripting without decayHEADtrunkmaster

N3517 (array subscripting without decay) has been added to C2y (via a
remote vote in May, not at a meeting).  Implement this in GCC.

The conceptual change, that the array subscripting operator [] no
longer involves an array operand decaying to a pointer, is something
GCC has done for a very long time.  The main effect in terms of what
is made possible in the language, subscripting a register array
(undefined behavior in C23 and before), was available as a GNU
extension, but only with constant indices.  There is also a new
constraint that array indices must not be negative when they are
integer constant expressions and the array operand has array type
(negative indices are fine with pointers) - an access out of bounds of
an array (even when contained within a larger object) has undefined
behavior at runtime when not a constraint violation.

Thus, the previous GCC extension is adapted to allow the cases of
register arrays not previously allowed, clearing DECL_REGISTER on them
as needed (similar to what is done with register declarations of
structures with volatile members) and restricting the pedwarn to
pedwarn_c23.  That pedwarn_c23 is also extended to cover the C23 case
of register compound literals (although not strictly needed since it
was undefined behavior rather than a constraint violation in C23).
The new error is added (only for flag_isoc2y) for negative array
indices with an operand of array type.

N3517 has some specific wording about the type of the result of
non-lvalue array element access.  It's unclear what's actually desired
there in the case where the array element is itself of array type; see
C23 issue 1001 regarding types of qualified members of rvalue
structures and unions more generally.  Rather than implementing the
specific wording about this in N3517, that is deferred until there's
an accepted resolution to issue 1001 and can be dealt with as part of
implementing such a resolution.

Nothing specific is done about the obsolescence in that paper of
writing index[array] or index[pointer] as opposed to array[index] or
pointer[index], although that seems like a reasonable enough thing to
warn about.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c/
	* c-typeck.cc (c_mark_addressable): New parameter
	override_register.
	(build_array_ref): Update calls to c_mark_addressable.  Give error
	in C2Y mode for negative array indices when array expression is an
	array not a pointer.  Use pedwarn_c23 for subscripting register
	array; diagnose that also for register compound literal.
	* c-tree.h (c_mark_addressable): Update prototype.

gcc/testsuite/
	* gcc.dg/c23-array-negative-1.c, gcc.dg/c23-register-array-1.c,
	gcc.dg/c23-register-array-2.c, gcc.dg/c23-register-array-3.c,
	gcc.dg/c23-register-array-4.c, gcc.dg/c2y-array-negative-1.c,
	gcc.dg/c2y-register-array-2.c, gcc.dg/c2y-register-array-3.c: New
	tests.

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