[Sema] Add -Wno-self-assign-overloaded

Summary:
It seems there isn't much enthusiasm for -wtest D45685.

This is more conservative version, which i had in the very first
revision of D44883, but that 'erroneously' got removed because of the review.

Based on some [irc] discussions, it must really be documented that
we want all the new diagnostics to have their own flags, to ease
rollouts, transitions, etc.

Please do note that i'm only adding -Wno-self-assign-overloaded,
but not -Wno-self-assign-field-overloaded, because i'm honestly
not aware of any false-positives from the -field variant,
but i can just as easily add it if wanted.
https://reviews.llvm.org/D44883#1068561

Reviewers: dblaikie, aaron.ballman, thakis, rjmccall, rsmith

Reviewed By: dblaikie

Subscribers: Quuxplusone, chandlerc, cfe-commits

Differential Revision: https://reviews.llvm.org/D45766