This commit adds a new type DynamicLookupInfo that provides information
about how a dynamic member lookup found a particular Decl. This is
needed to correctly handle KeyPath dynamic member lookups, but for now
just plumb it through everywhere.
The client usually cares about a subset of all expressions. A way to differentiate
them is by the protocols these expressions' types conform to. This patch allows
the request to add a list of protocol USRs so that the response only includes those
interested expressions that conform to any of the input protocols.
We also add a field to the response for each expression type to indicate the
conforming protocols names that were originally in the input list.
When an empty list of protocol USRs are given, we report all expressions' types
in the file like the old behavior.
rdar://35199889
`source.request.conformingmethods` is a new SourceKit request which
receives a source position and a list of protocol namses, returns a list
of methods whose return type conforms to the requested protocols.
rdar://problem/44699573
