[clang-format] Handle NullCoalescing and NullConditional operators in C#

Summary:
Disable merging of Type? into a single token.

Merge ?? ?. and ?[ into a single token.

Reviewers: krasimir, MyDeveloperDay

Reviewed By: krasimir

Subscribers: cfe-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D75368