[SourceKit] The initial implementation of range-info request.

[nkcsgexi]

Like cursor-info, range info (""source.request.cursorinfo"") answers some
questions clients have for a code snippet under selection, for instance, the type of a selected
expression. This commit implements this new quest kind and provides two
simple information about the selected code: (1) the kind of the
snippet, currently limited to single-statement and expression; and (2)
the type of the selected expression. Gradually, we will enrich the
response to provide more insight into the selected code snippet.

[nkcsgexi]

@swift-ci Please smoke test and merge

[modocache]

This is awesome, thanks!

Would it be possible to add documentation for new requests as they're added? SR-2117 tracks documenting the existing SourceKit request types. It would be great to keep that list of undocumented request types from growing. As a contributor I find it pretty difficult to add documentation for request types based on just their implementation and tests.

[nkcsgexi]

@modocache Sure, more available documentation is always a good idea. Will do!