[Ada] Improve handling of explicit by-reference mechanism

This improves the handling of an explicit by-reference passing mechanism
specified by means of the GNAT pragma Export_Function.  This device sort
of circumvents the rules of the language for the by-reference passing
mechanism and it's then up to the programmer to ensure that the actual
parameter is addressable; if it is not, the compiler will generate a
temporary around the call, thus effectively passing the actual by copy.

It turns out that the compiler was too conservative when determining
whether the actual parameter is addressable, in particular if it's a
component of a record type subject to a representation clause.

The change effectively moves this computation from the front-end to the
back-end, which has much more information on the layout and alignment
of types and thus can be less conservative.

2019-09-19  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch6.adb (Is_Legal_Copy): Also return false for an aliased
	formal and a formal passed by reference in convention Ada.  Add
	missing guard to the existing test on Is_Valued_Procedure.

From-SVN: r275931

gcc/ada/ChangeLog

+2019-09-19  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* exp_ch6.adb (Is_Legal_Copy): Also return false for an aliased
+	formal and a formal passed by reference in convention Ada.  Add
+	missing guard to the existing test on Is_Valued_Procedure.
+
 2019-09-19  Bob Duff  <duff@adacore.com>
 
 	* rtsfind.ads (RTE_Available): Improve comment.

gcc/ada/exp_ch6.adb

@@ -1859,12 +1859,16 @@ package body Exp_Ch6 is
          --  An attempt to copy a value of such a type can only occur if
          --  representation clauses give the actual a misaligned address.
 
-         if Is_By_Reference_Type (Etype (Formal)) then
+         if Is_By_Reference_Type (Etype (Formal))
+           or else Is_Aliased (Formal)
+           or else (Mechanism (Formal) = By_Reference
+                     and then not Has_Foreign_Convention (Subp))
+         then
 
             --  The actual may in fact be properly aligned but there is not
             --  enough front-end information to determine this. In that case
-            --  gigi will emit an error if a copy is not legal, or generate
-            --  the proper code.
+            --  gigi will emit an error or a warning if a copy is not legal,
+            --  or generate the proper code.
 
             return False;
 
@@ -1875,6 +1879,7 @@ package body Exp_Ch6 is
          --  be lurking.
 
          elsif Mechanism (Formal) = By_Reference
+           and then Ekind (Scope (Formal)) = E_Procedure
            and then Is_Valued_Procedure (Scope (Formal))
          then
             Error_Msg_N