Simplify PackedVector by removing user-defined special members that aren't any...

Simplify PackedVector by removing user-defined special members that aren't any different than the defaults

This causes the other special members (like move and copy construction,
and move assignment) to come through for free. Some code in clang was
depending on the (deprecated, in the original code) copy ctor. Now that
there's no user-defined special members, they're all available without
any deprecation concerns.

llvm-svn: 244835