Use a bit of relaxed constexpr to make FeatureBitset costant intializable

Use a bit of relaxed constexpr to make FeatureBitset costant intializable

This requires std::intializer_list to be a literal type, which it is
starting with C++14. The downside is that std::bitset is still not
constexpr-friendly so this change contains a re-implementation of most
of it.

Shrinks clang by ~60k.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369847 91177308-0d34-0410-b5e6-96231b3b80d8