swift234

[Diag] Make fixItReplace slightly smart

When replace something with a punctuator, we often prefer adding spaces around it.
For instance,

   func foo(): bar {}
   // fix it
   func foo() -> bar {}

In this case we want to add a space before '->', but not after that.

With this change, we can simply `fixItReplace(ColonLoc, " -> ")`.
`fixItReplace()` automatically adjust the spaces around it.