swift234

Reapply r29419:
Enhance fixItRemove() to be a bit more careful about what whitespace it leaves around: if the thing it is removing has leading and trailing whitespace already, this nukes an extra space to avoid leaving double spaces or incorrectly indented results.  

This includes an extra fix for looking off the start of a buffer, which extractText doesn't and can't handle.

This fixes <rdar://problem/21045509> Fixit deletes 'let' from non-binding 'if case let' statements, but leaves an extra space




Swift SVN r29449