clang-format-vsix: add a date stamp to the VSIX version number to ensure upgradability

clang-format-vsix: add a date stamp to the VSIX version number to ensure upgradability

Presently, the version number of the VSIX matches the LLVM version number. However, as this number doesn't change often, it means that as we release new versions of this VSIX, it will have the same version number, which means users must first uninstall the old version before installing the new one. With this change, we generate a 4th part to the version number that is a date stamp (year, month, day); for example: 4.0.0.161203.

Differential Revision: https://reviews.llvm.org/D27438

llvm-svn: 289909