[Core] Properly stop the ThreadPoolExecutor and use the ConcRT memory allocator.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@181279 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Initialize readers and writers on creation.

This makes the target handler a constructor argument because the constructor of
OutputELFWriter relies on it being initialized.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@181280 91177308-0d34-0410-b5e6-96231b3b80d8