[Ada] Suppress_Initialization not respected for private subtypes

[Pierre-Marie de Rodat]

The compiler fails to suppress initialization on a variable of a subtype
of a private type (such as System.Address) even though the subtype has
aspect Suppress_Initialization. This can lead to errors on object
declarations specified with Thread_Local_Storage when Initialize_Scalars
is applied (as well as leading to default initialization when it
shouldn't).

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

2019-08-12  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Pragma, Pragma_Suppress_Initialization):
	For private types, set the Suppress_Initialization flag on the
	Full_View of the entity rather than the entity's base type.

gcc/testsuite/

	* gnat.dg/suppress_initialization2.adb,
	gnat.dg/suppress_initialization2.ads: New testcase.
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