Resubmit an earlier patch of mine:

reduce the size of relevant "ghostly" sentinels
by a pointer.

This attempt now makes the compactification dependent
on the configure variable LLVM_COMPACT_SENTINELS
and should not cause any bootstrap failures for
llvm-gcc any more.

Please note that this is not yet the final version,
and (as settled with Chris) I shall take out the
autofoo/cmake portions in the next days.

This will also lose the assertability on sentinel
dereferencing and operator++, but that seems
an acceptable price to pay for the simplified
build logic.

llvm-svn: 79938