Unify lookup from within not-yet-defined defaulted special members: use common
code for handling triviality, deletedness and constexpr. Fix a few bugs in
these, particularly related to mutable members, and remove some dead code.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195809 91177308-0d34-0410-b5e6-96231b3b80d8