Ada: Fix variable initialized with if-expression not flagged as constant

Ada: Fix variable initialized with if-expression not flagged as constant

This is a regression present on the mainline and 15 branch: the -gnatwk
switch no longer flags a string variable initialized with an if-expression
as constant when it is not modified in the program.  The fix is to set the
Has_Initial_Value and Never_Set_In_Source flags earlier during analysis in
the Analyze_Object_Declaration procedure.

gcc/ada/
	PR ada/122640
	* sem_ch3.adb (Analyze_Object_Declaration): Set Is_True_Constant
	on entry for constants and Never_Set_In_Source in all cases.
	If an initialization expression is present, set Has_Initial_Value
	and Is_True_Constant on variables.

gcc/testsuite/
	* gnat.dg/warn34.adb: New test.