MS ABI: Simplify microsoft mangling of template instantiations

Use mangled template instantiation name as key for back references.
Templates have their own context for back references, so their mangling
is always the same regardless of context. This avoids mangling template
instantiations twice.

Patch by Agustín Bergé!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210416 91177308-0d34-0410-b5e6-96231b3b80d8