[DebugInfo] Handle complex expressions with spills in LiveDebugValues

In r369026 we disabled spill-recognition in LiveDebugValues for anything
that has a complex expression. This is because it's hard to recover the
complex expression once the spill location is baked into it.

This patch re-enables spill-recognition and slightly adjusts the DBG_VALUE
insts that LiveDebugValues tracks: instead of tracking the last DBG_VALUE
for a variable, it tracks the last _unspilt_ DBG_VALUE. The spill-restore
code is then able to access and copy the original complex expression; but
the rest of LiveDebugValues has to be aware of the slight semantic shift,
and produce a new spilt location if a spilt location is propagated between
blocks.

The test added produces an incorrect variable location (see FIXME), which
will be the subject of future work.

Differential Revision: https://reviews.llvm.org/D65368

llvm-svn: 369092