[unittests] ADT: silence -Wself-assign diagnostics

[unittests] ADT: silence -Wself-assign diagnostics

Summary:
D44883 extends -Wself-assign to also work on C++ classes.
In it's current state (as suggested by @rjmccall), it is not under it's own sub-group.
Since that diag is enabled by `-Wall`, stage2 testing showed that:
* It does not fire on any llvm code
* It does fire for these 3 unittests
* It does fire for libc++ tests

This diff simply silences those new warnings in llvm's unittests.
A similar diff will be needed for libcxx. (`libcxx/test/std/language.support/support.types/byteops/`, maybe something else)

Since i don't think we want to repeat rL322901, let's talk about it.
I've subscribed everyone who i think might be interested...

There are several ways forward:
* Not extend -Wself-assign, close D44883. Not very productive outcome i'd say.
* Keep D44883 in it's current state.
  Unless your custom overloaded operators do something unusual for when self-assigning,
  the warning is no less of a false-positive than the current -Wself-assign.
  Except for tests of course, there you'd want to silence it. The current suggestion is:
  ```
  S a;
  a = (S &)a;
  ```
* Split the diagnostic in two - `-Wself-assign-builtin` (i.e. what is `-Wself-assign` in trunk),
  and `-Wself-assign-overloaded` - the new part in D44883.
  Since, as i said, i'm not really sure why it would be less of a error than the current `-Wself-assign`,
  both would still be in `-Wall`. That way one could simply pass `-Wno-self-assign-overloaded` for all the tests.
  Pretty simple to do, and will surely work.
* Split the diagnostic in two - `-Wself-assign-trivial`, and `-Wself-assign-nontrivial`.
  The choice of which diag to emit would depend on trivial-ness of that particular operator.
  The current `-Wself-assign` would be `-Wself-assign-trivial`.
  https://godbolt.org/g/gwDASe - `A`, `B` and `C` case would be treated as trivial, and `D`, `E` and `F` as non-trivial.
  Will be the most complicated to implement.

Thoughts?

Reviewers: aaron.ballman, rsmith, rtrieu, rjmccall, dblaikie, atrick, gottesmm

Reviewed By: dblaikie

Subscribers: lebedev.ri, phosek, vsk, rnk, thakis, sammccall, mclow.lists, llvm-commits, rjmccall

Differential Revision: https://reviews.llvm.org/D45082

llvm-svn: 329491