Implement C++1y constant initializer rules: in a constant initializer for an
object x, x's subobjects can be constructed by constexpr constructor even if
they are of non-literal type, and can be read and written even though they're
not members of a constexpr object or temporary.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181506 91177308-0d34-0410-b5e6-96231b3b80d8