gccrs: Unify ASTValidation::visit for ExternalFunctionItem and Function

gccrs: Unify ASTValidation::visit for ExternalFunctionItem and Function

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit):
	Add external function validation support. Add ErrorCode::E0130.
	* parse/rust-parse-impl.h (Parser::parse_function): Parse
	external functions from `parse_function`.
	(Parser::parse_external_item): Clang format.
	(Parser::parse_pattern): Clang format.
	* parse/rust-parse.h: Add default parameter
	`is_external` in `parse_function`.

Signed-off-by: 0xn4utilus <gyanendrabanjare8@gmail.com>