[Ada] Fix validity checks on attribute 'Old prefix

[Pierre-Marie de Rodat]

Validity checks for 'Old prefixes (enabled by the -gnatVo switch) are
now executed when the 'Old prefix is evaluated, i.e. at the very
beginning of a subprogram and not when evaluating the postcondition.

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

2020-06-19  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): Call Ensure_Valid
	on the expression of an object declaration that captures the
	value of 'Old prefix.
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