swift234

[IDE] Code completion for Objective-C #keyPath expressions.

Implement code completion support for Objective-C #keyPath
expressions, using semantic analysis of the partially-typed keypath
argument to provide an appropriate set of results (i.e., just
properties and types).

This implements all of the necessary parts of SE-0062 / SR-1237 /
rdar://problem/25710611, although at some point I'd like to follow it
up with some warnings to help migrate existing string literals to