When performing template argument deduction for a non-reference

When performing template argument deduction for a non-reference
conversion function when we're binding the result to a reference, drop
cv-qualifiers on the type we're referring to, since we should be
deducing a type that can be adjusted (via cv-qualification) to the
requested type. Fixes PR9336, and the remaining Boost.Assign failure.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127117 91177308-0d34-0410-b5e6-96231b3b80d8