gccrs: ast-builder: Add methods for QualifiedPathInExpressions

gccrs: ast-builder: Add methods for QualifiedPathInExpressions

gcc/rust/ChangeLog:

	* ast/rust-ast-builder.cc (Builder::qualified_path_in_expression): New.
	(Builder::function): Change the return type.
	* ast/rust-ast-builder.h: Declare qualified_path_in_expression functions.
	* expand/rust-derive-debug.cc (DeriveDebug::stub_debug_fn): Adapt to new APIs.