Warning-free clang-format plugin install for VS 15.0

Warning-free clang-format plugin install for VS 15.0

With the new release of VS, it's required that all plugins migrate to
the new VSIX manifest format. The new format is backwards compatible
with all versions newer that Visual Studio 2012, so this migration
effectively drops support for older versions of the IDE.

It's also required that these new extensions are built with Visual
Studio 2017, so unfortunately it was necessary to migrate the project
and solution. Also removed COM references to EnvDTE and
Microsoft.VisualStudio.TextManager.Interop from the csproj, as they seem
to both be unnecessary and would trigger build warnings because of
changes to GAC.

Patch by Hugo Puhlmann!

Differential Revision: https://reviews.llvm.org/D31740

llvm-svn: 300225