gccrs: Replace reference to unique pointer with reference

gccrs: Replace reference to unique pointer with reference

Reference to unique pointers are a known anti pattern, only the element
shall be taken by reference instead of the whole wrapper.

gcc/rust/ChangeLog:

	* ast/rust-item.h: Change getter function prototype to return a
	reference directly instead of a reference to the wrapper type.
	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Fix
	the code to accept references instead.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::lower_self): Change
	function implementation to return a reference.
	* hir/rust-ast-lower-base.h: Accept a reference instead of a unique
	pointer reference.
	* resolve/rust-ast-resolve-item.cc (ResolveItem::visit): Adapt the code
	to a reference instead of a unique pointer.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>