Properly implement the part of C++ [over.match.funcs]p4 that treats

conversion functions as if their acting context were the class that
we're converting from (the implicit object argument's
type). Retroactively tweaking the implicit conversion sequence, as we
were trying to do before, breaks the invariants of that implicit
conversion sequence (e.g., the types and conversions don't match
up). Fixes <rdar://problem/8018274>.

llvm-svn: 111520