Merging r338602:

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r338602 | hans | 2018-08-01 19:51:23 +0200 (Wed, 01 Aug 2018) | 11 lines

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.
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_70@338674 91177308-0d34-0410-b5e6-96231b3b80d8