clang-format vs plugin: set version number from cmake (PR20307)

Previously it was hard-coded to 1.0, which meant the installer would
not install the plugin over previous versions.

This commit makes us use LLVM's major.minor.patch version from cmake,
or whatever CLANG_FORMAT_VS_VERSION is set to when configuring the build.

It's pretty dirty to update a configuration file in the source directory
from the cmake build like this. However, the plugin build is already
dirty in this regard since it builds in the source dir when visual studio,
and then copies out the resulting vsix.

llvm-svn: 213584