Issue Description:

As per discussion on Swift evolution (http://thread.gmane.org/gmane.comp.lang.swift.evolution/17575), Chris Lattner suggested it doesn't need to go through Swift evolution.

Interpolation of an Optional should create a warning and offer two fix-it suggestions (http://article.gmane.org/gmane.comp.lang.swift.evolution/20960):

a) Add redundant parentheses around the expression to silence the warning,
b) Add " ?? <#placeholder#>" for adding a default value.

I've originally thought I'd have a whack at it, but I won't have time until mid July to look around the compiler code, so I'm filing this as a bug report if anyone can get to it faster.