swift234

Initial stab at implementing string literal interpolation for simple expressions,
e.g. "foo is \(i+j)".  This implements rdar://11223686

Doug implemented all the hard parts of this.  I ripped out support for nested string
literals (i.e. string literals within an interpolated string), which simplified the
approach and defined away some problems with his patch in progress.  I plan a few refinements
on top of this basic patch.



Swift SVN r1738