gccrs: Dynamic dispatch with supertraits

gccrs: Dynamic dispatch with supertraits

gcc/rust/ChangeLog:

	* backend/rust-compile.cc:
	Modify compute_address_for_trait_item to support supertraits
	* typecheck/rust-tyty.cc:
	Remove auto

gcc/testsuite/ChangeLog:

	* rust/compile/trait13.rs:
	Add test for supertraits of supertraits
	* rust/compile/trait14.rs:
	Diamond problem with supertraits test
	* rust/execute/torture/trait14.rs:
	Add test for dynamic dispatch with supertraits
	* rust/execute/torture/trait15.rs:
	Add test for dynamic dispatch with generics
	* rust/execute/torture/trait16.rs:
	Add test for dynamic dispatch with lifetime params 1
	* rust/execute/torture/trait17.rs:
	Add test for dynamic dispatch with lifetime params 2
	* rust/execute/torture/trait18.rs:
	Add test for default implementations with dynamic dispatch and
	supertraits

Signed-off-by: Liam Naddell <liam.naddell@mail.utoronto.ca>