Fix correspondence between location/c and Location in typed/racket

[joergen7]

Checklist

• Bugfix
• Feature
• tests included
• documentation

See also: Issue #1415

In (untyped) racket, the XML collection uses a source struct whose fields start and stop follow the contract location/c. The location/c contract is defined as follows:

(or/c location? symbol? #f)

However, in typed/racket the aforementioned fields are typed as Location which represents only the struct but neither symbols nor false.

This pull request extends the typing of source's start and stop to entail both symbols and false typing them as

(U Location Symbol False)