[DebugInfo][InstrRef] Track variable assignments in out-of-scope blocks

DBG_INSTR_REF's and  DBG_VALUE's can end up in blocks that aren't in the
lexical scope of their variable. It's arguable as to what we should do
about this, however VarLocBasedLDV permits such variable locations to be
propagated, so let's allow it in InstrRefBasedLDV.

It's necessary for the modified test to work.

Differential Revision: https://reviews.llvm.org/D114578