Factors the cursor_info requests into a single API to asynchronously
make the sourcekitd cursor_info request and wrap the result for easier
consumption from Swift (but not other processing).
Also, cleanup the now unused extensions on Hover that were superceded by
symbolInfo.
Rework LineTable. Now that line table is just a collection of `Substring`.
Derive UTF16/UTF8 offset from corresponding view of the content String.
Added several methods to translate between line/UTF(8|16)column and
UTF8 offset.
Resolves: https://bugs.swift.org/browse/SR-9311