ada: Use static allocation for small dynamic string concatenations in more cases

ada: Use static allocation for small dynamic string concatenations in more cases

This lifts the limitation of the original implementation whereby the first
operand of the concatenation needs to have a length known at compiled time
in order for the static allocation to be used.

gcc/ada/

	* exp_ch4.adb (Expand_Concatenate): In the case where an operand
	does not have both bounds known at compile time, use nevertheless
	the low bound directly if it is known at compile time.
	Fold the conditional expression giving the low bound of the result
	in the general case if the low bound of all the operands are equal.