Simplify test/CodeGenCXX/microsoft-abi-vtables-single-inheritance.cpp by forcing VFTableBuilder invocation with virtual function calls

Previously the vftables were built at the end of the TU in a reverse-to-random order

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204455 91177308-0d34-0410-b5e6-96231b3b80d8