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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313640 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed