[Ada] Spurious visibility error on Declare_Expression with renames

[Pierre-Marie de Rodat]

The compiler rejects a declare expression whose declarations are only
object renaming declarations, when the expression references those
local objects.

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

gcc/ada/

	* sem_res.adb (Resolve_Declare_Expression): Retrieve the created
	block entity that is the scope of the local declarations, from
	either a local object declaration or an object renaming
	declaration. The block entity does not have an explicit
	declaration, but appears as the scope of all locally declared
	objects.
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