Use the multi-lockable form of std::lock for operator=

Use the multi-lockable form of std::lock for operator=

For = operators for lists that have mutexes, we were either
just taking the locks sequentially or hand-rolling a trick
to try to avoid lock inversion.  Use the std::lock mechanism
for this instead.

Differential Revision: https://reviews.llvm.org/D59957

llvm-svn: 357276