[SourceKit] Add an optional path and name to refactoring edits
Add two new fields to refactoring edits:
- A file path if the edit corresponds to a buffer other than the
original file
- A buffer name when the edit is actually source of generated buffer
Macro expansions allow the former as a macro could expand to member
attributes, which may eg. add accessors to each member. The attribute
itself is inside the expansion, but the edit is to the member in the
original source.
The latter will later allow clients to send requests with these names to
allow semantic functionality inside synthesized buffers.
