Fix RCS1197

[jamesHargreaves12]

An interpolated string can also be a verbatim string. Currently, The code fix for RCS1197 will not retain the verbatim @ when optimizing a string interpolation which means that the resulting code doesn't compile.

For example

stringBuilder.Append($@"{x:hh\:mm\:ss\.fff}"));

Becomes

stringBuilder.AppendFormat("{0:hh\:mm\:ss\.fff}",x));