MS ABI: Mangle variable templates properly

We wouldn't recognize variable templates as being templates leading us
to leave the template arguments off of the mangled name.  This would
allow two unrelated templates to map to the same mangled name.

N.B.  While MSVC doesn't support variable templates as of this date,
this mangling is the most likely thing they will choose to use.  Their
demangler can successfully demangle our manglings with the template
arguments shown.