swift234

- convert a memmove to a memcpy (even though they're the same on darwin).
- Switch LiteralChecks to hold a CanType to declare that it is canonicalized.
- Switch LiteralChecks from std::map to DenseMap.

I'm sure the perf of this doesn't actually matter, but std::map gives me the
hives on general principles :)



Swift SVN r4336