[DebugInfo][InstrRef] Terminate overlapping variable fragments

If we have a variable where its fragments are split into overlapping
segments:

    DBG_VALUE $ax, $noreg, !123, !DIExpression(DW_OP_LLVM_fragment_0, 16)
    ...
    DBG_VALUE $eax, $noreg, !123, !DIExpression(DW_OP_LLVM_fragment_0, 32)

we should only propagate the most recently assigned fragment out of a
block. LiveDebugValues only deals with live-in variable locations, as
overlaps within blocks is DbgEntityHistoryCalculators domain.

InstrRefBasedLDV has kept the accumulateFragmentMap method from
VarLocBasedLDV, we just need it to recognise DBG_INSTR_REFs. Once it's
produced a mapping of variable / fragments to the overlapped variable /
fragments, VLocTracker uses it to identify when a debug instruction needs
to terminate the other parts it overlaps with. The test is updated for
some standard "InstrRef picks different registers" variation, and the
order of some unrelated DBG_VALUEs changes.

Differential Revision: https://reviews.llvm.org/D114603