[Ada] Spurious error in nested generic containing expression function

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch removes spurious visibility errors from the instantiation of a
generic package nested within another generic, when the inner package
contains an expression function that is the completion of a visible
function of that package, and the expression includes an object of a
tagged type local to the generic.

No small example available.

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

2017-09-29  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch6.adb (Analyze_Expression_Function): Do not emit freeze nodes
	for types in expression if the function is within a generic unit.
	* sem_res.adb (Resolve): In a generic context do not freeze an
	expression, unless it is an entity. This exception is solely for the
	purpose of detecting illegal uses of deferred constants in generic
	units.
	* sem_res.adb: Minor reformatting.

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