[-cxx-abi microsoft] Create backrefs for <unnamed-type-`id'>

It wasn't possible for an anonymous type to show up inside of function arguments.
However, decltype (which MSVC added support for in 2010) makes this
possible.  Further, backrefs to these anonymous types can now be formed.

This fixes PR18022.

N.B. We do not, and very likely _will not_, support MSVC's bug where
subsequent typedefs of anonymous types leak into the linkage name; this
is a gross violation of the ABI.  A warning should be introduced to
inform our users of this particular shortcoming.

llvm-svn: 195669