Fix some type-traits (is_assignable, etc) dealing with classes that take non-const references as 'right hand side'. Add tests. Fixes PR# 20836

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@218286 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed