Clean up type flags for overloaded Neon builtins. No functional change.

Clean up type flags for overloaded Neon builtins.  No functional change.

This patch just adds a simple NeonTypeFlags class to replace the various
hardcoded constants that had been used until now.  Unfortunately I couldn't
figure out a good way to avoid duplicating that class between clang and
TableGen, but since it's small and rarely changes, that's not so bad.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144054 91177308-0d34-0410-b5e6-96231b3b80d8