gdb: Convert language la_post_parser field to a method
    
    This commit changes the language_data::la_post_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 (resolve): Rename to ada_language::post_parser.
    	(ada_language_data): Delete la_post_parser initializer.
    	(ada_language::post_parser): New member function.
    	* c-lang.c (c_language_data): Delete la_post_parser initializer.
    	(cplus_language_data): Likewise.
    	(asm_language_data): Likewise.
    	(minimal_language_data): Likewise.
    	* d-lang.c (d_language_data): Likewise.
    	* f-lang.c (f_language_data): Likewise.
    	* go-lang.c (go_language_data): Likewise.
    	* language.c (unknown_language_data): Likewise.
    	(auto_language_data): Likewise.
    	* language.h (language_data): Delete la_post_parser field.
    	(language_defn::post_parser): New member function.
    	* m2-lang.c (m2_language_data): Delete la_post_parser initializer.
    	* objc-lang.c (objc_language_data): Likewise.
    	* opencl-lang.c (opencl_language_data): Likewise.
    	* p-lang.c (pascal_language_data): Likewise.
    	* parse.c (parse_exp_in_context): Update call to post_parser.
    	(null_post_parser): Delete definition.
    	* parser-defs.h (null_post_parser): Delete declaration.
    	* rust-lang.c (rust_language_data): Delete la_post_parser
    	initializer.