Follow-up to r300225: update ClangFormat.sln to VS2017

This got lost in the previous patch somehow.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300226 91177308-0d34-0410-b5e6-96231b3b80d8

tools/clang-format-vs/ClangFormat.sln

 
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 14
-VisualStudioVersion = 14.0.25420.1
+# Visual Studio 15
+VisualStudioVersion = 15.0.26228.12
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ClangFormat", "ClangFormat\ClangFormat.csproj", "{7FD1783E-2D31-4D05-BF23-6EBE1B42B608}"
 EndProject