gdb: Convert language la_parser field to a method

This commit changes the language_data::la_parser function pointer
member variable into a member function of language_defn.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (parse): Rename to ada_language::parser.
	(ada_language_data): Delete la_parser initializer.
	(ada_language::parser): New member function, implementation from
	parse.
	* c-lang.c (c_language_data): Delete la_parser initializer.
	(cplus_language_data): Likewise.
	(asm_language_data): Likewise.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	(d_language::parser): New member function.
	* f-lang.c (f_language_data): Delete la_parser initializer.
	(f_language::parser): New member function.
	* go-lang.c (go_language_data): Delete la_parser initializer.
	(go_language::parser): New member function.
	* language.c (unk_lang_parser): Delete.
	(language_defn::parser): Define new member function.
	(unknown_language_data): Delete la_parser initializer.
	(unknown_language::parser): New member function.
	(auto_language_data): Delete la_parser initializer.
	(auto_language::parser): New member function.
	* language.h (language_data): Delete la_parser field.
	(language_defn::parser): Declare new member function.
	* m2-lang.c (m2_language_data): Delete la_parser initializer.
	(m2_language::parser): New member function.
	* objc-lang.c (objc_language_data): Delete la_parser initializer.
	* opencl-lang.c (opencl_language_data): Likewise.
	* p-lang.c (pascal_language_data): Likewise.
	(pascal_language::parser): New member function.
	* parse.c (parse_exp_in_context): Update call to parser.
	* rust-lang.c (rust_language_data): Delete la_parser initializer.
	(rust_language::parser): New member function.