Add a special diagnostic for when the parser is not making progress in loops

[RikkiGibson]

See IsMakingProgress(ref int):

roslyn/src/Compilers/CSharp/Portable/Parser/SyntaxParser.cs

Line 1107 in 1ccaa91

protected bool IsMakingProgress(ref int lastTokenPosition)

This method Debug.Asserts so that we know there's an infinite loop bug in the parser in debug mode. In release mode, it just returns 'false' so that the related loop terminates and compilation finishes (expected to finish with syntax errors.)

I think we should consider making it so this method also adds a special diagnostic which requests the user to report an issue, so that when users inadvertently put the parser into an unexpected no-progress state, we can find out about it and address any underlying issues.

/cc @jcouv