OS X: fix the Xcode debugserver lookup code when LLDB.framework does not contain a debugserver

LLDB recently started supporting LLDB.framework without a
debugserver in it.  When that happens, the Xcode-included debugserver
is searched for and used.  This change fixes the code that looks for
Xcode when the housing process is not Xcode.  In particular, this
addresses the problem where python is running the test suite and
the LLDB.framework does not contain a debugserver.

llvm-svn: 252059