Implement basic code completion support for #selector with property
getters/setters. The vast majority of this implementation comes from
Alex Hoppen (@ahoppen), with only a handful of my own tweaks. Alex has
more interesting ideas on improving this that I wasn't quite ready to
commit to, so this is more basic than the overall goal.
When we're code completing a postfix or dot expression inside the
subexpression of an #selector expression, prefer compound function
names. This helps us write, e.g.,
#selector(UIView.
and get completions such as "insertSubview(_:aboveSubview:)". Fixes
rdar://problem/24470075.
