gdb: Convert language la_is_string_type_p field to a method
    
    This commit changes the language_data::la_is_string_type_p 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_language_data): Delete la_is_string_type_p
    	initializer.
    	(ada_language::is_string_type_p): New member function.
    	* c-lang.c (c_language_data): Delete la_is_string_type_p
    	initializer.
    	(cplus_language_data): Likewise.
    	(asm_language_data): Likewise.
    	(minimal_language_data): Likewise.
    	* d-lang.c (d_language_data): Likewise.
    	* f-lang.c (f_is_string_type_p): Delete function, implementation
    	moved to f_language::is_string_type_p.
    	(f_language_data): Delete la_is_string_type_p initializer.
    	(f_language::is_string_type_p): New member function,
    	implementation from f_is_string_type_p.
    	* go-lang.c (go_is_string_type_p): Delete function, implementation
    	moved to go_language::is_string_type_p.
    	(go_language_data): Delete la_is_string_type_p initializer.
    	(go_language::is_string_type_p): New member function,
    	implementation from go_is_string_type_p.
    	* language.c (language_defn::is_string_type_p): Define new member
    	function.
    	(default_is_string_type_p): Make static, add comment copied from
    	header file.
    	(unknown_language_data): Delete la_is_string_type_p initializer.
    	(unknown_language::is_string_type_p): New member function.
    	(auto_language_data): Delete la_is_string_type_p initializer.
    	(auto_language::is_string_type_p): New member function.
    	* language.h (language_data): Delete la_is_string_type_p field.
    	(language_defn::is_string_type_p): Declare new function.
    	(default_is_string_type_p): Delete desclaration, move comment to
    	definition.
    	* m2-lang.c (m2_is_string_type_p): Delete function, implementation
    	moved to m2_language::is_string_type_p.
    	(m2_language_data): Delete la_is_string_type_p initializer.
    	(m2_language::is_string_type_p): New member function,
    	implementation from m2_is_string_type_p.
    	* objc-lang.c (objc_language_data): Delete la_is_string_type_p
    	initializer.
    	* opencl-lang.c (opencl_language_data): Likewise.
    	* p-lang.c (pascal_is_string_type_p): Delete function,
    	implementation moved to pascal_language::is_string_type_p.
    	(pascal_language_data): Delete la_is_string_type_p initializer.
    	(pascal_language::is_string_type_p): New member function,
    	implementation from pascal_is_string_type_p.
    	* rust-lang.c (rust_is_string_type_p): Delete function,
    	implementation moved to rust_language::is_string_type_p.
    	(rust_language_data): Delete la_is_string_type_p initializer.
    	(rust_language::is_string_type_p): New member function,
    	implementation from rust_is_string_type_p.
    	* valprint.c (val_print_scalar_or_string_type_p): Update call to
    	is_string_type_p.