Add ToString to FSharp.Compiler.Text.StringText

There exists no method to retrieve the original source string because the 'String' property on StringText is never exposed in ISourceText

Author: deviousasti

src/utils/prim-lexing.fs

@@ -53,6 +53,7 @@ type StringText(str: string) =
 
     override __.GetHashCode() = str.GetHashCode()
     override __.Equals(obj: obj) = str.Equals(obj)
+    override __.ToString() = str
 
     interface ISourceText with