Refine memory buffer for importing shared cache objc class list

In https://reviews.llvm.org/D118972 I increased this buffer to be
big enough to import 261,144 classes but this is a lot more than
we currently have, an allocating a too-large buffer can add memory
pressure even if it's only for a short time.  Reduce the size of
this memory buffer to big enough to import 163,840 classes.  I'll
probably move to a scheme where we read the objc classes in chunks,
with a smaller buffer and multiple inferior function calls.

rdar://91275493