acquired only after checking if the ivar shared pointer was already
filled in. But when I assign an UnwindPlan object to the shared
pointer, I assign an empty object and then fill it in. That leaves
a window where another thread could get the shared pointer to the
empty (but quickly being-filled-in) object and lead to a crash.
Also two changes from Greg for correctness on the TestMultipleDebuggers
test case.
<rdar://problem/30564102>
llvm-svn: 296084
1e7ecd3e64