[ADT] restrict bit_cast to trivially-constructible To

Summary: As discussed in r341853 by blaikie, the reinterpret_cast was technically an aliasing violation. Restrict our bit_cast implementation to To which are trivially-constructible (and note the existing restriction to constexpr). Once we move to C++17 we can use a version of bit_cast without these restrictions, or if we care we can SFINAE a different implementation when To isn't trivially-constructible.

Reviewers: dblaikie, rsmith

Subscribers: dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D52332

llvm-svn: 342710