[constexpr] Support for constant evaluation of __builtin_memcpy and

__builtin_memmove (in non-type-punning cases).

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: 338455