Revert r338455 "[constexpr] Support for constant evaluation of...

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
> trivially-copyable types.
>
> __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.

llvm-svn: 338602