gcc/: Merge definitions of array_type_nelts_top

There were two identical definitions, and none of them are available
where they are needed for implementing a number-of-elements-of
operator.  Merge them, and provide the single definition in
gcc/tree.{h,cc}, where it's available for that operator, which will be
added in a following commit.

gcc/ChangeLog:

	* tree.h (array_type_nelts_top)
	* tree.cc (array_type_nelts_top):
	Define function (moved from gcc/cp/).

gcc/cp/ChangeLog:

	* cp-tree.h (array_type_nelts_top)
	* tree.cc (array_type_nelts_top):
	Remove function (move to gcc/).

gcc/rust/ChangeLog:

	* backend/rust-tree.h (array_type_nelts_top)
	* backend/rust-tree.cc (array_type_nelts_top):
	Remove function.

Signed-off-by: Alejandro Colomar <alx@kernel.org>