gdb/ada: Update some predicate functions to return bool

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.