This change permits UnresolvedDotExpr to have both a name and a base that are implicit, but a valid DotLoc, and to treat that DotLoc as the node’s location. It then changes the generation of string interpolation code so that `$stringInterpolation.appendInterpolation` references have a DotLoc corresponding to the backslash in the string literal.
This makes it possible for `ExprContextAnalyzer` in IDE to correctly detect when you are code-completing in a string interpolation and treat it as an `appendInterpolation` call.