Move value_true to value.h

I noticed that value_true is declared in language.h and defined in
language.c.  However, as part of the value API, I think it would be
better in one of those files.  And, because it is very short, I
changed it to be an inline function in value.h.  I've also removed a
comment from the implementation, on the basis that it seems obsolete
-- if the change it suggests was needed, it probably would have been
done by now; and if it is needed in the future, odds are it would be
done differently anyway.

Finally, this patch also changes value_true and value_logical_not to
return a bool, and updates some uses.

gdb/opencl-lang.c

@@ -748,7 +748,7 @@ opencl_logical_binop_operation::evaluate (struct type *expect_type,
       /* For scalar built-in types, only evaluate the right
 	 hand operand if the left hand operand compares
 	 unequal(&&)/equal(||) to 0.  */
-      int tmp = value_logical_not (arg1);
+      bool tmp = value_logical_not (arg1);
 
       if (op == BINOP_LOGICAL_OR)
 	tmp = !tmp;