[Ada] Set Etype on rewriteen Max_Queue_Length expressions

[Pierre-Marie de Rodat]

Rewriting of Max_Queue_Length expression into N_Integer_Literal should probably
be done in expansion and not in analysis, but anyway it should not strip the
expression from its Etype because backends (e.g. GNATprove) expect that Etype
to be present.

No frontend test is provided, because GNAT doesn't care about the missing
Etype decoration. This patch allows to simplify AST processing in GNATprove.

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

2018-05-31  Piotr Trojanek  <troja...@adacore.com>

gcc/ada/

        * sem_prag.adb (Analyze_Pragma): Set Etype on the rewritten
        Max_Queue_Length expression.
--- gcc/ada/sem_prag.adb
+++ gcc/ada/sem_prag.adb
@@ -18833,6 +18833,7 @@ package body Sem_Prag is
 
             if Nkind (Arg) /= N_Integer_Literal then
                Rewrite (Arg, Make_Integer_Literal (Sloc (Arg), Val));
+               Set_Etype (Arg, Etype (Original_Node (Arg)));
             end if;
 
             Record_Rep_Item (Entry_Id, N);