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.

This commit repplies r24305 with two fixes:

  - It fixes the computation of spare bits for unusual
    integer types to use the already-agreed-upon type
    size instead of recomputing it.  This fixes the
    i386 stdlib build.  Joe and I agreed that we should
    also change the size to use the LLVM alloc size
    instead of the next power of 2, but this patch
    does not do that yet.

  - It changes the spare bits in function types back
    to the empty set.  I'll be changing this in a
    follow-up, but it needs to be tied to runtime
    changes.  This fixes the regression test failures.

Swift SVN r24324