Previously `SDDbgValue`s used the general allocator that lives for all
of `SelectionDAG`. Instead, give them their own allocator, and reset it
whenever `SDDbgInfo::clear()` is called, plugging a spiritual leak.
This drops `SelectionDAGBuilder::visitIntrinsicCall()` off of my heap
profile (was at around 2% of `llc` for codegen of `-flto -g`). Thanks
to Pete Cooper for spotting the problem and suggesting the fix.
llvm-svn: 237998