[Ada] Legality checks on the use of incomplete types.

[Arnaud Charlet <charlet at adacore dot com>]

This patch completes the implementation of the rules given in RM 3.10.1, and
allows the detection of additional illegal uses of incomplete types in 
formal parts and return types of access_to_subprogram types.

Examples in ACATS test B3A1A04.

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

2009-04-17  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch3.adb (Access_Subprogram_Definition): Additional checks on
	illegal uses of incomplete types in formal parts and return types.

	* sem_ch6.adb (Process_Formals): Taft-amendment types are legal in
	access to subprograms.

	* sem_ch7.adb (Uninstall_Declarations): diagnose attempts to use
	Taft-amendment types as the return type of an access_to_function type.

	* freeze.adb (Freeze_Entity): Remove tests on formals of an incomplete
	type for access_to_subprograms. The check is performed on package exit.

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