[Ada] Use high-level Present instead of low-level equality test

[Pierre-Marie de Rodat]

Routine Present for Unit_Name_Type was introduced precisely to avoid
equality tests with No_Unit_Name. Code cleanup only; semantics is
unaffected.

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

gcc/ada/

        * lib-load.adb, lib-writ.adb, lib.adb, par-load.adb,
        rtsfind.adb, sem_ch10.adb: Use Present where possible.
diff --git a/gcc/ada/lib-load.adb b/gcc/ada/lib-load.adb
--- a/gcc/ada/lib-load.adb
+++ b/gcc/ada/lib-load.adb
@@ -551,7 +551,7 @@ package body Lib.Load is
       --  Note: Unit_Name (Main_Unit) is not set if we are parsing gnat.adc.
 
       if Present (Error_Node)
-        and then Unit_Name (Main_Unit) /= No_Unit_Name
+        and then Present (Unit_Name (Main_Unit))
       then
          --  It seems like In_Extended_Main_Source_Unit (Error_Node) would
          --  do the trick here, but that's wrong, it is much too early to


diff --git a/gcc/ada/lib-writ.adb b/gcc/ada/lib-writ.adb
--- a/gcc/ada/lib-writ.adb
+++ b/gcc/ada/lib-writ.adb
@@ -837,7 +837,7 @@ package body Lib.Writ is
             --  preprocessing data and definition files, there is no Unit_Name,
             --  check for that first.
 
-            if Unit_Name (J) /= No_Unit_Name
+            if Present (Unit_Name (J))
               and then (With_Flags (J) or else Unit_Name (J) = Pname)
             then
                Num_Withs := Num_Withs + 1;


diff --git a/gcc/ada/lib.adb b/gcc/ada/lib.adb
--- a/gcc/ada/lib.adb
+++ b/gcc/ada/lib.adb
@@ -275,7 +275,7 @@ package body Lib is
    begin
       --  First unregister the old name, if any
 
-      if Old_N /= No_Unit_Name and then Unit_Names.Get (Old_N) = U then
+      if Present (Old_N) and then Unit_Names.Get (Old_N) = U then
          Unit_Names.Set (Old_N, No_Unit);
       end if;
 


diff --git a/gcc/ada/par-load.adb b/gcc/ada/par-load.adb
--- a/gcc/ada/par-load.adb
+++ b/gcc/ada/par-load.adb
@@ -318,7 +318,7 @@ begin
 
       Spec_Name := Get_Parent_Spec_Name (Unit_Name (Cur_Unum));
 
-      if Spec_Name /= No_Unit_Name then
+      if Present (Spec_Name) then
          Unum :=
            Load_Unit
              (Load_Name  => Spec_Name,


diff --git a/gcc/ada/rtsfind.adb b/gcc/ada/rtsfind.adb
--- a/gcc/ada/rtsfind.adb
+++ b/gcc/ada/rtsfind.adb
@@ -423,7 +423,7 @@ package body Rtsfind is
                          (Unit_Name (Current_Sem_Unit));
 
                   begin
-                     if Parent_Name /= No_Unit_Name then
+                     if Present (Parent_Name) then
                         Get_Name_String (Parent_Name);
 
                         declare


diff --git a/gcc/ada/sem_ch10.adb b/gcc/ada/sem_ch10.adb
--- a/gcc/ada/sem_ch10.adb
+++ b/gcc/ada/sem_ch10.adb
@@ -768,7 +768,7 @@ package body Sem_Ch10 is
             Unum := Get_Cunit_Unit_Number (N);
             Par_Spec_Name := Get_Parent_Spec_Name (Unit_Name (Unum));
 
-            if Par_Spec_Name /= No_Unit_Name then
+            if Present (Par_Spec_Name) then
                Unum :=
                  Load_Unit
                    (Load_Name  => Par_Spec_Name,