gdb: Convert language la_word_break_characters field to a method

gdb: Convert language la_word_break_characters field to a method

This commit changes the language_data::la_word_break_characters
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 (ada_get_gdb_completer_word_break_characters): Delete.
	(ada_language_data): Delete la_word_break_characters initializer.
	(ada_language::word_break_characters): New member function.
	* c-lang.c (c_language_data): Delete la_word_break_characters
	initializer.
	(cplus_language_data): Likewise.
	(asm_language_data): Likewise.
	(minimal_language_data): Likewise.
	* completer.c: Update global comment.
	(advance_to_expression_complete_word_point): Update call to
	word_break_characters.
	(complete_files_symbols): Likewise.
	(complete_line_internal_1): Likewise.
	(default_completer_handle_brkchars): Likewise.
	(skip_quoted_chars): Likewise.
	* d-lang.c (d_language_data): Delete la_word_break_characters
	initializer.
	* f-lang.c (f_word_break_characters): Delete.
	(f_language_data): Delete la_word_break_characters initializer.
	(f_language::word_break_characters): New member function.
	* go-lang.c (go_language_data): Delete la_word_break_characters
	initializer.
	* language.c (unknown_language_data): Likewise.
	(auto_language_data): Likewise.
	* language.h (default_word_break_characters): Move declaration to
	earlier in the file.
	(language_data): Delete la_word_break_characters field.
	(language_defn::word_break_characters): New member function.
	* m2-lang.c (m2_language_data): Delete la_word_break_characters
	initializer.
	* objc-lang.c (objc_language_data): Likewise.
	* opencl-lang.c (opencl_language_data): Likewise.
	* p-lang.c (pascal_language_data): Likewise.
	* rust-lang.c (rust_language_data): Likewise.