[ms-cxxabi] Fix r182865 to have an actual Itanium-style key function

Peter pointed out that C::f() is not a key function.  C's key function
is actually C::~C().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182866 91177308-0d34-0410-b5e6-96231b3b80d8