ada: Transform Length attribute references for non-Strict overflow mode.

The non-strict overflow checking code does a better job of eliminating
overflow checks if given an expression consisting only of predefined
operators (including relationals), literals, identifiers, and conditional
expressions. If it is both feasible and useful, rewrite a
Length attribute reference as such an expression. "Feasible" means
"index type is same type as attribute reference type, so we can rewrite without
using type conversions". "Useful" means "Overflow_Mode is something other than
Strict, so there is value in making overflow check elimination easier".

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): If it makes sense
	to do so, then rewrite a Length attribute reference as an
	equivalent conditional expression.