[ELF] Make addressOfAtom thread safe.

The expression _atomToAddressMap[atom] may modify _atomToAddressMap.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@182793 91177308-0d34-0410-b5e6-96231b3b80d8
[Core] Add parallel_for_each.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@182794 91177308-0d34-0410-b5e6-96231b3b80d8
Parallel write.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@182795 91177308-0d34-0410-b5e6-96231b3b80d8