c++: Fix bootstrap on 32-bit hosts [PR91828]

c++: Fix bootstrap on 32-bit hosts [PR91828]

Using the releasing_vec op[] with an int index was breaking on 32-bit hosts
because of ambiguity with the built-in operator and the conversion
function.  Since the built-in operator has a ptrdiff_t, this was fine on
64-bit targets where ptrdiff_t is larger than int, but broke on 32-bit
targets where it's the same as int, making the conversion for that argument
better than the member function.  Fixed by changing the member function to
also use ptrdiff_t for the index.

gcc/cp/ChangeLog:

	* cp-tree.h (releasing_vec::operator[]): Change parameter type to
	ptrdiff_t.