Treat C# `using` as a control statement

Contributed by jbcoe!

Summary: Unless SpaceBeforeParensOptions is set to SBPO_Never, a space will be put between `using` and `(` in C# code.

Reviewers: klimek, MyDeveloperDay, krasimir

Reviewed By: krasimir

Subscribers: MyDeveloperDay, cfe-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D72144