[Ada] Don't constant-fold renamed qualified expressions

[Pierre-Marie de Rodat]

In a case like:

   X : Integer := 1;
   Y : Integer renames Integer'(X);

the value of Y is changed by any subsequent assignments to X. Thus,
replacing the use of X with a literal 1 would be a mistake.

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

gcc/ada/

	* exp_ch2.adb (Expand_Entity_Reference): A new local predicate
	Is_Object_Renaming_Name indicates whether a given expression
	occurs (after looking through qualified expressions and type
	conversions) as the name of an object renaming declaration. If
	Current_Value is available but this new predicate is True, then
	ignore the availability of Current_Value.
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