Add GetLeftSideOfDot from Roslyn

[Tim-Pohlmann]

GetLeftSideOfDot would be very useful in a lot of places and should be brought over to our code base.

Example use case: https://github.com/SonarSource/sonar-dotnet/blob/95278ed63294c01e3f90ca9d4b8d82255477aebf/analyzers/src/SonarAnalyzer.CSharp/Rules/RedundantCast.cs#L146-L154