Fix thread race between SectionPiece's OutputOff and Live members

Summary:
As reported in bug 35788, rL316280 reintroduces a race between two
members of SectionPiece, which share the same 64 bit memory location.

To fix the race, check the hash before checking the Live member, as
suggested by Rafael.

Reviewers: ruiu, rafael

Reviewed By: ruiu

Subscribers: smeenai, emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D41884