Add fix-it for a regex that starts or ends with a space

[hamishknight]

Description

We ought to provide a fix-it for a regex such as / x/ or / x/ to use an extended literal instead e.g #/ x/#. For the space at the start we could also suggest to insert a backslash. This depends on #1463 being merged first.

Steps to Reproduce

No response

[ahoppen]

rdar://107469888