[Ada] Add continuation message when others choice not allowed

[Pierre-Marie de Rodat]

In a context where the bounds of an array aggregate are not known, the
aggregate is not allowed to include an "others" choice. The error
message given by GNAT now includes a suggestion to qualify the aggregate
with a constrained subtype to fix the issue.

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

gcc/ada/

	* sem_aggr.adb (Resolve_Array_Aggregate): Improve error message.
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