[clang-format] Add basic support for formatting C# files

[clang-format] Add basic support for formatting C# files

Summary:

This revision adds basic support for formatting C# files with clang-format, I know the barrier to entry is high here  so I'm sending this revision in to test the water as to whether this might be something we'd consider landing.

Tracking in Bugzilla as:
https://bugs.llvm.org/show_bug.cgi?id=40850

Justification:
C# code just looks ugly in comparison to the C++ code in our source tree which is clang-formatted.

I've struggled with Visual Studio reformatting to get a clean and consistent style, I want to format our C# code on saving like I do now for C++ and i want it to have the same style as defined in our .clang-format file, so it consistent as it can be with C++.  (Braces/Breaking/Spaces/Indent etc..)

Using clang format without this patch leaves the code in a bad state, sometimes when the BreakStringLiterals is set, it  fails to compile.

Mostly the C# is similar to Java, except instead of JavaAnnotations I try to reuse the TT_AttributeSquare.

Almost the most valuable portion is to have a new Language in order to partition the configuration for C# within a common .clang-format file, with the auto detection on the .cs extension. But there are other C# specific styles that could be added later if this is accepted. in particular how  `{ set;get }` is formatted.

Reviewers: djasper, klimek, krasimir, benhamilton, JonasToth

Reviewed By: klimek

Subscribers: llvm-commits, mgorny, jdoerfert, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D58404

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@356662 91177308-0d34-0410-b5e6-96231b3b80d8