Add missing visitors for AST::Function.

Add missing visitors for AST::Function.

To use AST::Function instead of AST::TraitItemFunc and
AST::TraitItemMethod, we need to provide similar visitors during
lowering and resolving phase.

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-implitem.cc (ASTLowerTraitItem::visit):
	Provide visitor for AST::Function.
	* hir/rust-ast-lower-implitem.h:
	Likewise.
	* resolve/rust-ast-resolve-implitem.h:
	Likewise.
	* resolve/rust-ast-resolve-item.cc (ResolveTraitItems::visit):
	Likewise.
	* resolve/rust-ast-resolve-item.h:
	Likewise.

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