[clang-format] Recognize C# named argument colons as a token type

Summary:
No longer merge 'name' and ':' into a single token.

Ensure that line breaks cannot be placed before or after a named-argument colon.

Ensure that no space is inserted before a named-argument colon.

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: cfe-commits, MyDeveloperDay

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D75244