HasSource::source_old -> HasSource::source for places where proc-macros were special...

HasSource::source_old -> HasSource::source for places where proc-macros were special cased

In #6901 some special case handling for proc-macros was introduced to
prevent panicing as they have no AST. Now the new HasSource::source
method is used that returns an option.

Generally this was a pretty trivial change, the only thing of much
interest is that `hir::MacroDef` now implements `TryToNav` not `ToNav`
as this allows us to handle `HasSource::source` now returning an option.