[clang-format] Add more support for C# 8 nullables

This adds support for the null-coalescing assignment and null-forgiving
operators.

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-coalescing-operator

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-forgiving

Reviewed By: krasimir, curdeius

Differential Revision: https://reviews.llvm.org/D101702