[ms-cxxabi] There are no key functions in the Microsoft C++ ABI

MSVC's class data is always comdat, so clang's should always be
linkonce_odr in LLVM IR.

Reviewers: pcc

Differential Revision: http://llvm-reviews.chandlerc.com/D838

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182865 91177308-0d34-0410-b5e6-96231b3b80d8