This forces iterpolated expressions to be single-line, so that newline
literals within the bounds of two string delimiters can be seen as
verbatime newlines in the resulting string.
Edge case: in the case of a line comment, it's not possible to keep
this as a single line expression.
These are kept as multi-line expressions.
Also:
* Remove `ForceWrap`, this node is not used.
* Rename `StringConstant` -> `StringChars`
Daniel Chao