Update resolver to use `AST::Function` instead of `AST::ExternalFunctionItem`

Update resolver to use `AST::Function` instead of `AST::ExternalFunctionItem`

gcc/rust/ChangeLog:

	* checks/errors/rust-feature-gate.cc (FeatureGate::visit):
	Check if function is_external or not.
	* hir/rust-ast-lower-extern.h: Use AST::Function
	instead of AST::ExternalFunctionItem.
	* parse/rust-parse-impl.h (Parser::parse_external_item):
	Likewise.
	(Parser::parse_pattern): Fix clang format.
	* resolve/rust-ast-resolve-implitem.h: Likewise.
	* resolve/rust-ast-resolve-item.cc (ResolveExternItem::visit):
	Likewise.
	* resolve/rust-ast-resolve-item.h: Likewise.
	* resolve/rust-default-resolver.cc (DefaultResolver::visit):
	Check if param has_pattern before using get_pattern.

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