[clang-format] Recognize C# nullable types

Summary:
Do not confuse C# nullable types with conditional expressions.

Do not put a space before the `?` in `[access-modifier] Type? variableName;`

Reviewers: krasimir, MyDeveloperDay

Reviewed By: krasimir, MyDeveloperDay

Subscribers: cfe-commits, MyDeveloperDay

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D75261