[llvm] [cmake] Use current directory in GenerateVersionFromVCS

Find dependent scripts of GenerateVersionFromVCS in current directory
rather than ../../cmake/modules.  I do not see any reason why the former
would not work and The latter is incorrect when GenerateVersionFromVCS
is used from install directory (i.e. in stand-alone builds).

Differential Revision: https://reviews.llvm.org/D57996

llvm-svn: 353674