[Ada] Keep assertions in internal units enabled for GNATprove

[Pierre-Marie de Rodat]

In GNATprove mode the assertion policy is now always enabled, even when
analysing internal units. Otherwise, assertion expressions (e.g.
Default_Initial_Condition) in internal units (e.g. Ada.Text_IO)
disappear in the semantic analysis phase of the frontend and the
GNATprove backend can't see them.

No frontend test provided, because only the GNATprove backend is
affected (and there appear to be no difference in the output with -gnatG
switch, because the expansion of Default_Initial_Condition is not
attached to the AST).

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

2019-07-04  Piotr Trojanek  <troja...@adacore.com>

gcc/ada/

        * opt.adb (Set_Config_Switches): Keep assertions policy as
        enabled when analysing internal units in GNATprove mode.
--- gcc/ada/opt.adb
+++ gcc/ada/opt.adb
@@ -248,7 +248,13 @@ package body Opt is
             SPARK_Mode_Pragma        := SPARK_Mode_Pragma_Config;
 
          else
-            if GNAT_Mode_Config then
+            --  In GNATprove mode assertions should be always enabled, even
+            --  when analysing internal units.
+
+            if GNATprove_Mode then
+               pragma Assert (Assertions_Enabled);
+               null;
+            elsif GNAT_Mode_Config then
                Assertions_Enabled    := Assertions_Enabled_Config;
             else
                Assertions_Enabled    := False;