[clang-format] C# short ternary operator misinterpreted as a CSharpNullable

Refactor the CSharpNullable assignment code to be a little easier to read (Honestly I don't like it when an if expression get really long and complicated).
Handle the case where '?' is actually a ternary operator.

Fixes: #58067

Reviewed By: owenpan, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D148473