gccrs: Fix undefined behaviour issues on macos

This adds missing copy constructors to HIR::PathExprSegment which were
wrongly defaulting to empty vectors when apply specified generic arguments
to method calls.

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-expr.h: Add const `get_method_name`.
	* hir/tree/rust-hir-full-decls.h (struct GenericArgs): Move from `struct`...
	(class GenericArgs): ...to `class`.
	* hir/tree/rust-hir-path.h (struct GenericArgs): Likewise.
	(class GenericArgs): Clear `type_args` in copy constructor.
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): Reorder
	debug print.
	* typecheck/rust-tyty.h: Add default constructors for `SubstitutionArgumentMappings`.