[Ada] Present and No functions for type Uint

[Ada] Present and No functions for type Uint

gcc/ada/

	* uintp.ads, uintp.adb (Present, No): New functions for
	comparing with No_Uint.
	* checks.adb, einfo-utils.adb, exp_aggr.adb, exp_attr.adb,
	exp_ch3.adb, exp_ch4.adb, exp_dbug.adb, exp_disp.adb,
	exp_util.adb, repinfo.adb, repinfo-input.adb, scn.adb,
	sem_attr.adb, sem_ch13.adb, sem_eval.adb, sem_util.adb,
	sinfo-utils.adb, treepr.adb: Use Present (...) instead of "...
	/= No_Uint", and No (...) instead of "... = No_Uint".