[Ada] Better error message on missing parentheses

[Pierre-Marie de Rodat via Gcc-patches]

Adapt the test to issue a different error message when it is likely that
an if-expression is suspected, but parentheses are missing. This makes
the test more in line with its comment.

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

gcc/ada/

        * par-ch4.adb (P_Primary): Adapt test for getting error message
        on missing parentheses.
diff --git a/gcc/ada/par-ch4.adb b/gcc/ada/par-ch4.adb
--- a/gcc/ada/par-ch4.adb
+++ b/gcc/ada/par-ch4.adb
@@ -2892,8 +2892,10 @@ package body Ch4 is
                if Token_Is_At_Start_Of_Line
                  and then not
                    (Ada_Version >= Ada_2012
-                     and then Style_Check_Indentation /= 0
-                     and then Start_Column rem Style_Check_Indentation /= 0)
+                      and then
+                        (Style_Check_Indentation = 0
+                           or else
+                             Start_Column rem Style_Check_Indentation /= 0))
                then
                   Error_Msg_AP ("missing operand");
                   return Error;