[Ada] Present and No functions for type Uint

[Pierre-Marie de Rodat]

Declare Present and No functions for type Uint, analogous to other types
such as Node_Id, and use them as appropriate.

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

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".

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