Enhanced the ObjC DynamicCheckerFunction to test for "object responds to selector" as well as

"object borked"...  Also made the error when the checker fails reflect this fact rather than
report a crash at 0x0.

Also a little cleanup:
- StopInfoMachException had a redundant copy of the description string.
- ThreadPlanCallFunction had a redundant copy of the thread, and had a 
copy of the process that it didn't really need.

llvm-svn: 143419