Re-land r313400 "[DebugInfo] Insert DW_OP_deref when spilling indirect DBG_VALUEs"

I forgot to zero out the BitVector when reusing it between UserValues.

Later uses of the same location number for a different UserValue would
falsely indicate that they were spilled. Usually this would lead to
incorrect debug info, but in some cases they would indicate something
nonsensical like a memory location based on a vector register (Q8 on
ARM).

llvm-svn: 313640