[Ada] Clean up Get_Index_Bounds

[Pierre-Marie de Rodat]

Replace some calls to procedure Get_Index_Bounds with calls
to the function with the same name. Not all calls are replaced
(some seem clearer as procedure calls).

Change names to be more consistent with the RM.

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

gcc/ada/

        * checks.adb, exp_aggr.adb, exp_ch5.adb, freeze.adb,
        sem_util.adb, sem_util.ads: Change L and H to be First and Last,
        to match the attributes in the RM. Change calls from procedure
        to function where appropriate.

patch.diff.gz
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