When determining whether the two types involved in reference binding

When determining whether the two types involved in reference binding
are reference-compatible, reference-related, etc., do not complete the
type of the reference itself because it is not necessary to determine
well-formedness of the program. Complete the type that we are binding
to, since that can affect whether we know about a derived-to-base
conversion.

Fixes PR7080.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103220 91177308-0d34-0410-b5e6-96231b3b80d8