It isn't that easy, as no string transformation can deal with invalid escape sequences like a raw literal can (nor can it disable interpolation).
But I agree that we could do more with functions than we do now:
Given the discussion about Compile-Time Constant Expressions for Swift - #105 by marcrasi, I don't think it makes much sense to have two kinds of raw string literals.
The multiline variant wouldn't be really raw anyways, and the processing can happen explicitly:
let rawString = '
This is a newline,
this \n is not.
Look, two spaces!
'.whitespaceStrippedAsYouLike