Commas, angle brackets dismiss signature help in strings

TL;DR

We believe that all of C#'s trigger characters should probably be considered retrigger characters.

Context

We (@jessetrinity, @sana-ajani, @amcasey and I on the TypeScript team) are trying to tackle the Visual Studio side of https://github.com/Microsoft/TypeScript/issues/1150. We noticed that the most "intuitive" implementation actually results in the undesirable behavior highlighted above. When checking to see what C# does, we saw the same behavior. Rather than diverging, we wanted to consult with Roslyn first to verify whether this is a bug.

Details

Version Used: Latest

Steps to Reproduce:

1. Write a method Foo accepting a string
2. Trigger signature help session within a string literal being passed to Foo
3. Enter a comma character within the string (,)

Expected Behavior:

Signature help remains

Actual Behavior:

Signature help is dismissed