gdb/ada: Update some predicate functions to return bool

A later commit would like to make use of a pointer to the function
ada_is_string_type, however, this will require the function to return
a bool (so the signature matches).

As the ada_is_string_type is a predicate function, and its return
value is only ever used as either true or false, then this commit
updates the function to return a bool.

As a consequence ada_is_character_type needs to change too.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_is_character_type): Change return type to bool.
	(ada_is_string_type): Likewise.
	* ada-lang.h (ada_is_character_type): Update declaration
	(ada_is_string_type): Likewise.

gdb/ada-lang.c

@@ -9405,7 +9405,7 @@ value_val_atr (struct type *type, struct value *arg)
    [At the moment, this is true only for Character and Wide_Character;
    It is a heuristic test that could stand improvement].  */
 
-int
+bool
 ada_is_character_type (struct type *type)
 {
   const char *name;
@@ -9413,7 +9413,7 @@ ada_is_character_type (struct type *type)
   /* If the type code says it's a character, then assume it really is,
      and don't check any further.  */
   if (TYPE_CODE (type) == TYPE_CODE_CHAR)
-    return 1;
+    return true;
   
   /* Otherwise, assume it's a character type iff it is a discrete type
      with a known character type name.  */
@@ -9429,7 +9429,7 @@ ada_is_character_type (struct type *type)
 
 /* True if TYPE appears to be an Ada string type.  */
 
-int
+bool
 ada_is_string_type (struct type *type)
 {
   type = ada_check_typedef (type);
@@ -9444,7 +9444,7 @@ ada_is_string_type (struct type *type)
       return ada_is_character_type (elttype);
     }
   else
-    return 0;
+    return false;
 }
 
 /* The compiler sometimes provides a parallel XVS type for a given