[Sema] Extend -Wself-assign and -Wself-assign-field to warn on overloaded self-assignment (classes)

Summary:
This has just bit me, so i though it would be nice to avoid that next time :)
Motivational case:

https://godbolt.org/g/cq9UNk

Basically, it's likely to happen if you don't like shadowing issues,
and use -Wshadow and friends. And it won't be diagnosed by clang.

The reason is, these self-assign diagnostics only work for builtin assignment
operators. Which makes sense, one could have a very special operator=,
that does something unusual in case of self-assignment,
so it may make sense to not warn on that.

But while it may be intentional in some cases, it may be a bug in other cases,
so it would be really great to have some diagnostic about it...

Reviewers: aaron.ballman, rsmith, rtrieu, nikola, rjmccall, dblaikie

Reviewed By: rjmccall

Subscribers: EricWF, lebedev.ri, thakis, Quuxplusone, cfe-commits

Differential Revision: https://reviews.llvm.org/D44883