CodeGen: Make string tests exercise the MS ABI

r204562 unwittingly failed tests for some bots.  Make those tests work
with both the Itanium and MS ABIs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204566 91177308-0d34-0410-b5e6-96231b3b80d8