Prevent getting immutable context with classic nr

Prevent getting immutable context with classic nr

Immutable name resolution context is not initialized when the classic
name resolution is in use. It can therefore not be used, the getter would
error out.

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-path.cc (TypeCheckExpr::resolve_root_path):
	Only get immutable name resolution context when name resolution 2.0 is
	used.
	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::resolve_root_path):
	Likewise.

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