[clang-format] Do not merge target-name and : for C# attributes

Summary:
Re-use token type `TT_AttributeColon` for C# attribute target colons.

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: MyDeveloperDay, cfe-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D75465