DebugInfo: constify the AddressPool MCSymbol pointers

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185188 91177308-0d34-0410-b5e6-96231b3b80d8
DebugInfo: Simplify the AddressPool representation

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185189 91177308-0d34-0410-b5e6-96231b3b80d8