[Ada] Fix missing minus sign in literal translation

[Pierre-Marie de Rodat]

When translating literals to big reals, the compiler would forget about
the minus sign and turn a negative number into a positive one.

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

gcc/ada/

        * sem_res.adb (Resolve): Insert minus sign if needed.
diff --git a/gcc/ada/sem_res.adb b/gcc/ada/sem_res.adb
--- a/gcc/ada/sem_res.adb
+++ b/gcc/ada/sem_res.adb
@@ -2934,6 +2934,11 @@ package body Sem_Res is
                      else
                         UI_Image (Norm_Num (Expr_Value_R (Expr)), Decimal);
                         Start_String;
+
+                        if UR_Is_Negative (Expr_Value_R (Expr)) then
+                           Store_String_Chars ("-");
+                        end if;
+
                         Store_String_Chars
                           (UI_Image_Buffer (1 .. UI_Image_Length));
                         Param1 := Make_String_Literal (Loc, End_String);