Rename global constants from I to J. No functional changes.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-18  Claire Dross  <dross@adacore.com>

gcc/ada/

	* libgnat/a-cofuma.adb (Remove, Elements_Equal_Except,
	Keys_Included, Keys_Included_Except): Rename loop indexes and
	global constants from I to J.