[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  <dismu...@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.
--- gcc/ada/sem_prag.adb
+++ gcc/ada/sem_prag.adb
@@ -24169,7 +24169,7 @@ package body Sem_Prag is
                   Error_Pragma_Arg
                     ("argument of pragma% cannot be an incomplete type", Arg1);
                else
-                  Set_Suppress_Initialization (Full_View (Base_Type (E)));
+                  Set_Suppress_Initialization (Full_View (E));
                end if;
 
             --  For first subtype, set flag on base type

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/suppress_initialization2.adb
@@ -0,0 +1,5 @@
+package body Suppress_Initialization2 is
+
+   procedure Dummy is null;
+
+end Suppress_Initialization2;

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/suppress_initialization2.ads
@@ -0,0 +1,13 @@
+pragma Initialize_Scalars;
+
+with System;
+
+package Suppress_Initialization2 is
+
+   subtype Sub_Addr is System.Address with Suppress_Initialization;
+
+   O : Sub_Addr with Thread_Local_Storage;  -- OK: no error should be reported
+
+   procedure Dummy;
+
+end Suppress_Initialization2;