Do not make an unnecessary copy of DenseMap.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198035 91177308-0d34-0410-b5e6-96231b3b80d8
Simplify. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198036 91177308-0d34-0410-b5e6-96231b3b80d8
Micro-optimize Resolver::markLive().

This patch eliminates one std::set lookup per a function call.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198037 91177308-0d34-0410-b5e6-96231b3b80d8