Disable MSVC's warning about noreturn destructors

Disable MSVC's warning about noreturn destructors

This warning fires on:
  Operator::~Operator() {
    llvm_unreachable("should never destroy an Operator");
  }

That seems like a false positive.  I don't see any good way to silence
the warning here, so I'm disabling it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173455 91177308-0d34-0410-b5e6-96231b3b80d8