When completing in the only expression of closure, use the return type
of the closure as the type context for the code-completion. However,
since code-completion may be on an incomplete input, we only use the
return type to improve the quality of the result, not to mark it
invalid, since (a) we may add another statement afterwards, or (b) if
the context type is Void it doesn't need to match the value.
Instead of re-typechecking parsed expression, find typechecked
expression that corresponds to the parsed expression from the
typechecked decl context, because the sub expressions of the parsed
expression can be weirdly mutated/replaced by decl context typechecking.
rdar://problem/48141174
`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
