One more cleanup to lldb version printing

With this patch LLDB_VERSION_STRING replaces "lldb version x.x.x" if it is set. This allows builds to not display the open source version numbers if the people making the distribution overrides the LLDB_VERSION_STRING.

Since LLDB_VERSION_STRING is always overridden on Darwin, this means the first line of lldb -version on Darwin is:

lldb-360.99.0 (<repo path> revision <revision>)

llvm-svn: 286899