MS ABI: Stick internal vftables in a comdat if they have RTTI data

Previously, we would have a private backing variable and an internal
alias pointing at it.

However, -fdata-sections only fires if a global variable has non-private
linkage.  This means that an unreferenced vftable wouldn't get
discarded, bloating the object file.

Instead, stick the backing variable in a comdat even if the alias has
internal linkage.  This will allow the linker to drop the vftable if it
is unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212901 91177308-0d34-0410-b5e6-96231b3b80d8