[-cxx-abi microsoft] Remove ArgIndex, we handle all template argument kinds!

TemplateExpansion cannot happen here because MSVC doesn't mangle
anything but the fully substituted template arguments.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189325 91177308-0d34-0410-b5e6-96231b3b80d8