[DWARF parser] DWARFDebugFrame: use unique_ptr instead of raw pointer

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207459 91177308-0d34-0410-b5e6-96231b3b80d8
AddressPool::HasBeenUsed: Add comment explaining the use-case for this flag.

Based on code review by Eric Christopher on r207323

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207460 91177308-0d34-0410-b5e6-96231b3b80d8