Allow epilogue_begin to be emitted when generating DWARF

We identify epilogue code by looking for instructions tagged
with FrameDestroy.

A function may have more than one epilogue, e.g., because of early
returns or code duplicated during optimization. We need only track
the current block, and emit epilogie_begin at most once per block.

We reduce the number of entries in the line table by combining
epilogue_begin with other flags instead of emitting a separate
entry just for epilogue_begin.

Reviewed By: dblaikie, aprantl

Differential Revision: https://reviews.llvm.org/D133376