[ADT] bit_cast: check for is_trivially_copyable more portably

Summary:
It turns out that isPodLike isn't a good workaround for is_trivially_copyable for bit_cast's purpose. In D51872 Louis points out that tuple and pair really aren't a good fit, and for bit_cast I want to capture array. This patch instead checks is_trivially_copyable directly in bit_cast for all but GCC 4.x. In GCC 4.x developers only check for sizeof match, which means any mistake they make will succeed locally and fail on the bots. Realistically that's few developers and they'll be left behind once we upgrade past C++11.

This will allow using bit_cast with std::array.

Subscribers: dexonsmith, llvm-commits, ldionne, rsmith

Differential Revision: https://reviews.llvm.org/D51888

llvm-svn: 341865