[debugserver] Add --version/-V command line option to debugserver.

When not running under a TTY the output is buffered and not flushed
before debugserver exits which makes it impossible to parse the version
string. This adds a -V/--version command that just prints the version to
stdout and exits with an exit code zero.

Differential revision: https://reviews.llvm.org/D68156

llvm-svn: 373127