swift234

Switch IRGen to use ClusteredBitVector.

IRGen uses a typedef, SpareBitVector, for its principal
purpose of tracking spare bits.  Other uses should not
use this typedef, and I've tried to follow that, but I
did this rewrite mostly with sed and may have missed
some fixups.

This should be almost completely NFC.  There may be
some subtle changes in spare bits for witness tables
and other off-beat pointer types.  I also fixed a bug
where IRGen thought that thin functions were two
pointers wide, but this wouldn't have affected anything
because we never store thin functions anyway, since
they're not a valid AST type.

Swift SVN r24305