[libcxx][test] Silence -Wself-assign diagnostics

Summary:
D44883 extends -Wself-assign to also work on C++ classes.
These new warnings pop up in the test suite, so they have to be silenced.

Please refer to the D45082 for disscussion on whether this is the right way to solve this.

Testing: ninja check-libcxx check-libcxxabi in stage-2 build.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: Quuxplusone, cfe-commits

Differential Revision: https://reviews.llvm.org/D45128