Raw string interpolation has proven to be a bit trickier to
implement, since we don’t want to treat `#` as a delimiter
(since it’s used in so many other ways in Swift, for example
for `#if`, `#available`, etc.).
This patch contains a “quick fix” for supporting interpolated
raw strings that contain a single expression per interpolation.
A proper fix should be developed as soon as possible, but this
unblocks using Splash to highlight most raw string interpolated
code samples, for now.
dddf418cea