<rdar://problem/12350715>

Modify UnwindLLDB::SearchForSavedLocationForRegister so if the register
save locations for a register mid-stack is in another register (or in the
same register, indicating the reg wasn't modified in this frame), don't
return that as a found location.  Keep iterating down the array of frames
until a concrete location/value for the register is found, or until we
get to frame 0 where the reg value can be used as-is.

If lldb was trying to backtrace a program that blew out its stack via
recursion and the unwind instructions had some kind of
this-reg-is-saved-in-that-reg instruction, lldb would revert to doing
a recursive search for a concrete value and blow out its own stack.