[Ada] Add pragma Preelaborable_Initialization to Stream_IO.File_Type

[Pierre-Marie de Rodat]

Pragma Preelaborable_Initialization is added to type File_Type in
package Ada.Streams.Stream_IO, per the Binding Interpretation of
AI12-0102. No other source or tool changes are required.

No test needed.

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

2019-10-10  Gary Dismukes  <dismu...@adacore.com>

gcc/ada/

        * libgnat/a-ststio.ads (File_Type): Apply pragma
        Preelaborable_Initialization to the type.
--- gcc/ada/libgnat/a-ststio.ads
+++ gcc/ada/libgnat/a-ststio.ads
@@ -42,6 +42,7 @@ package Ada.Streams.Stream_IO is
    type Stream_Access is access all Root_Stream_Type'Class;
 
    type File_Type is limited private with Default_Initial_Condition;
+   pragma Preelaborable_Initialization (File_Type);
 
    type File_Mode is (In_File, Out_File, Append_File);