Fix various issues in std::any and the related tests.

* Fix self-swap. Patch from Casey Carter.

* Remove workarounds and tests for types with deleted move constructors. This
  was originally added as part of a LWG proposed resolution that has since
  changed.

* Re-apply most recent PR for LWG 2769.

* Re-apply most recent PR for LWG 2754. Specifically fix the SFINAE checks to
  use the decayed type.

* Fix tests to allow moved-from std::any's to have a non-empty state. This is
  the behavior of MSVC's std::any.

* Various whitespace and test fixes.

llvm-svn: 283606