[Ada] Add utility to preanalyze assert expression without forcing its type

[Ada] Add utility to preanalyze assert expression without forcing its type

In SPARK loop and subprogram variants we now allow expressions of any
discrete type or of Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer
type. This requires a variant of Preanalyze_Assert_Expression that
doesn't force the expression to be of a particular type, similar to the
existing variant of Analyze_And_Resolve.

gcc/ada/

	* sem_ch3.ads, sem_ch3.adb (Preanalyze_Assert_Expression): Add a
	variant that doesn't force preanalysis to yield a specific type.