gccrs: Parse trait functions as `AST::Function`.

To use AST::Function for trait functions, we can parse trait functions
using available parse_function().

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_trait_item):
	Use parse_function() to parse trait functions.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>