build_llvm_package.bat: Build teh clang-format plugin separately

build_llvm_package.bat: Build teh clang-format plugin separately

In r293373 we switched the build to linking dynamically against the
Universal CRT and include the redistributables in the installer.

However, clang-format.exe is copied into the vsix and needs to be
statically linked. This commit makes us build the plugin in a separate
step that uses static linking.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294513 91177308-0d34-0410-b5e6-96231b3b80d8