Fix MSVC build broken by r207580

Seems MSVC wants to be able to codegen inline-definitions of virtual
functions even in TUs that don't define the key function - and it's well
within its rights to do so.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207581 91177308-0d34-0410-b5e6-96231b3b80d8