Support C++ single-quote digit separators

[jwosty]

For example, see this C++ code for which I am trying to generate bindings: https://github.com/facebook/yoga/blob/be00354b7194bf182c9b66584f64c83e248d3007/yoga/style/StyleValueHandle.h#L55

Would be nice if the P/Invoke generator supported this syntax correctly. Currently it emits the literal as-is (with single quotes in it), but it should either strip those out, or if you want to get fancy, replace them with C#'s own digit separator (_).