[Clang] -Wself-assign should be enabled by default

[philnik777]

Currently, Clang doesn't seem to enable -Wself-assign by default without a good reason I can see. Anything this diagnoses is almost certainly a bug. This doesn't warn on weird user-defined operators either, since there is -Wself-assign-overloaded for that case.

[AaronBallman]

Currently, Clang doesn't seem to enable -Wself-assign by default without a good reason I can see.

IIRC the reason it's not enabled by default is because it's a somewhat common pattern used in older code bases to silence "maybe unused" diagnostics. Whether that is still the case today is another matter, some research would be required. Unfortunately, I can't find a way to get sourcegraph to help; they use Re2 syntax which doesn't support backreferences.