[Ada] Fix ineffective -gnatyN for separate compilation units

[Pierre-Marie de Rodat]

This fixes a recent regression introduced in the machinery giving style
warnings: the -gnatyN switch no longer cancels an earlier -gnaty switch
for separate compilation units.

Running this command:

  gcc -c slib.adb -gnatyaAbcefhiIklmM25OnprStux -gnatyN

On the following sources:

package Slib is
   procedure I_Am_Short;
end Slib;

with Ada.Text_IO; use Ada.Text_IO;

package body Slib is
   --  have a long line here as well.
   procedure                I_Am_Short is               separate;
end Slib;

separate (Slib)

procedure I_Am_Short is
begin
    Put_Line (".......................................................");
end I_Am_Short;

Should execute silently.

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

2019-09-17  Eric Botcazou  <ebotca...@adacore.com>

gcc/ada/

        * sem.adb (Do_Analyze): Save Style_Check_Max_Line_Length on
        entry and restore it on exit instead of recomputing it.
--- gcc/ada/sem.adb
+++ gcc/ada/sem.adb
@@ -1360,7 +1360,8 @@ package body Sem is
          --  unconditionally, and has no restore mechanism, because it is
          --  intended as a lowest-level Pure package.
 
-         Saved_ML : constant Int := Style_Max_Line_Length;
+         Saved_ML  : constant Int     := Style_Max_Line_Length;
+         Saved_CML : constant Boolean := Style_Check_Max_Line_Length;
 
          List : Elist_Id;
 
@@ -1395,7 +1396,7 @@ package body Sem is
          Restore_Scope_Stack  (List);
          Restore_Ghost_Region (Saved_GM, Saved_IGR);
          Style_Max_Line_Length := Saved_ML;
-         Style_Check_Max_Line_Length := Style_Max_Line_Length /= 0;
+         Style_Check_Max_Line_Length := Saved_CML;
       end Do_Analyze;
 
       --  Local variables