gdb: Convert language la_pass_by_reference field to a method
This commit changes the language_data::la_pass_by_reference function
pointer member variable into a member function of language_defn.
The interesting thing in this commit is that I have removed the
default_pass_by_reference function entirely. This function only ever
returned a language_pass_by_ref_info struct in its default state, so
all uses of this function can be replaced by just default
initialisation of a language_pass_by_ref_info variable.
There should be no user visible changes after this commit.