clang-format vs plugin support for Visual Studio 2017

clang-format vs plugin support for Visual Studio 2017

With the release of Visual Studio 2017, we need to at the very least
claim support for it in the current manifest file. With the changes
introducted in this patch we can install the extension again, but a
warning message will be shown stating that it's not supported
(https://twitter.com/parsley72/status/846558416751411200).

To get the rid of the warning more work is necessary, as VS 2017 changed
some things about extensions, see more here:
https://docs.microsoft.com/en-us/visualstudio/extensibility/how-to-migrate-extensibility-projects-to-visual-studio-2017.
While working on those changes, it has been suggested in the
mail list that this first patch is integrated in the meantime.

Patch by Hugo Puhlmann!

Differential Revision: https://reviews.llvm.org/D31522

llvm-svn: 299210