[Ada] Avoid crash in GNATprove_Mode on allocator inside type

[Pierre-Marie de Rodat]

In the special mode for GNATprove, subtypes should be declared for
allocators when constraints are used. This was done previously but it
does not work inside spec expressions, as the declaration is not
inserted and analyzed in the AST in that case, leading to a later crash
on an incomplete entity. Thus, no declaration should be created in such
a case, letting GNATprove later reject such code due to the use of an
allocator in an interfering context.

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

2019-08-13  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_ch4.adb (Analyze_Allocator): Do not insert subtype
	declaration for allocator inside a spec expression.

gcc/testsuite/

	* gnat.dg/allocator2.adb, gnat.dg/allocator2.ads: New testcase.
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