Bring back APInt self-move assignment check for MSVC only

Summary:
It was removed about a year ago in r300477. Bring it back, along with
its unittest, when the MSVC STL is in use. The MSVC STL performs
self-assignment in std::shuffle. These days, llvm::sort calls
std::shuffle when expensive checks are enabled to help find
non-determinism bugs.

Reviewers: craig.topper, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46028

llvm-svn: 330776