SDAG: Give SDDbgValues their own allocator (and reset it)

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237998 91177308-0d34-0410-b5e6-96231b3b80d8