Revert r338455 "[constexpr] Support for constant evaluation of builtin_memcpy and builtin_memmove (in non-type-punning cases)."
It caused asserts during Chromium builds, see reply on the cfe-commits thread.
This is intended to permit libc++ to make std::copy etc constexpr
without sacrificing the optimization that uses memcpy on
builtin_strcpy and builtin_wcscpy are not handled by this change.
They'd be straightforward to add, but we haven't encountered a need for
them just yet.