ada: Compiler crash on container aggregate association with nonstatic key choice

ada: Compiler crash on container aggregate association with nonstatic key choice

The compiler blows up on a container aggregate with a container element
association that has a key_choice given by a nonstatic key expression.
This happens in the size computation for the aggregate due to calling
Update_Choices with the nonstatic expression.  The fix is simply to
condition the call to Update_Choices on whether the choice expression
is static.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Build_Container_Aggr_Code.Build_Size_Expr): In the case
	of an association with a single choice, only call Update_Choices when
	the choice expression is nonstatic.