[LLDB][Minidump] Fix bug in generating 64b memory minidumps (#141995)

In #129307, we introduced read write in chunks, and during the final
revision of the PR I changed the behavior for 64b memory regions and did
not test an actual 64b memory range.

This caused LLDB to crash whenever we generated a 64b memory region. 

64b regions has been a problem in testing for some time as it's a waste
of test resources to generation a 5gb+ Minidump. I will work with
@clayborg and @labath to come up with a way to specify creating a 64b
list instead of a 32b list (likely via the yamilizer).

Author: Jlalond

lldb/source/Plugins/ObjectFile/Minidump/MinidumpFileBuilder.cpp

@@ -1170,7 +1170,6 @@ MinidumpFileBuilder::AddMemoryList_64(std::vector<CoreFileMemoryRange> &ranges,
               "(%" PRIx64 "bytes) "
               "[%" PRIx64 ", %" PRIx64 ")",
               region_index, ranges.size(), size, addr, addr + size);
-    ++region_index;
 
     progress.Increment(1, "Adding Memory Range " + core_range.Dump());
     uint64_t bytes_read = 0;
@@ -1186,6 +1185,8 @@ MinidumpFileBuilder::AddMemoryList_64(std::vector<CoreFileMemoryRange> &ranges,
       cleanup_required = true;
       descriptors[region_index].DataSize = bytes_read;
     }
+
+    ++region_index;
   }
 
   // Early return if there is no cleanup needed.