[benchmark] Disable CMake get_git_version

Disabled CMake get_git_version as it is meaningless for this in-tree
build, and hardcoded a null version.

Not using get_git_version avoids a refresh of the git index that is
executed by get_git_version. Refreshing the index can take a
considerable amount of time if the index needs to be refreshed
(particularly with the mono repo). This situation can arise when
building shared source on a host in VMs.

Differential Revision: https://reviews.llvm.org/D63925

llvm-svn: 364742