swift234

add a few "optimized" runtime entrypoints for swift_getTupleTypeMetadata
that handle the 2/3 element cases specially.  These are not actually
optimized at the moment (they just call into swift_getTupleTypeMetadata)
but this could be done at some point.

This is a win for a couple reasons: this reduces the amount of code generated
inline and it allows swift_getTupleTypeMetadata2/3 to be marked readnone,
enabling CSE.  As a driveby, optimize metadata refs of zero element tuples
to directly use _TMdT_, eliminating a branch from swift_getTupleTypeMetadata.



Swift SVN r5070