We shouldn't save g_dummy_target_sp. Other code will simply call Destroy() on it.

TestBackticksWithoutATarget.BackticksWithNoTargetTestCase was calling
GetDummyTarget() when executing for x86_64. When performing session
tearDown, it would get destroyed (and everything would be invalid (arch,
etc).

Then the test would run for i386. The dummy target wasn't being
reinitialized and was invalid. lldb complained that 'current process state
is unsuitable for expression parsing'.

llvm-svn: 156994