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

Author: jasonmolenda

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp

@@ -2010,7 +2010,8 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::UpdateISAToDescriptorMap() {
     return DescriptorMapUpdateResult::Fail();
 
   // The number of entries to pre-allocate room for.
-  const uint32_t max_num_classes = 256 * 1024;
+  // Each entry is (addrsize + 4) bytes
+  const uint32_t max_num_classes = 163840;
 
   UtilityFunction *get_class_info_code = GetClassInfoUtilityFunction(exe_ctx);
   if (!get_class_info_code) {