[clang-format] Handle C# generic type constraints

Summary:
Treat each C# generic type constraint, `where T: ...`, as a line.

Add C# keyword: where

Add Token Types: CSharpGenericTypeConstraint, CSharpGenericTypeConstraintColon, CSharpGenericTypeConstraintComma.

This patch does not wrap generic type constraints well, that will be addressed in a follow up patch.

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: cfe-commits, MyDeveloperDay

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D76367