swift234

Use metatype pointers as generic type arguments.  Movements
towards optimizing generic calls to derive things from the
'this' pointer, which is actually crucial for virtual
dispatch (to get all methods to agree about how the
implicit arguments are passed).  Fix a number of assorted
bugs in metadata emission.  Lots of assorted enhancements.

This was proving surprisingly difficult to actually tease
apart into smaller patches.

Swift SVN r2927