[lldb] Use the non-locking variant of objc_copyRealizedClassList

Avoid standing the Objective-C runtime lock by calling
objc_copyRealizedClassList_nolock instead of objc_copyRealizedClassList.

We already guarantee that no other threads can run while we're running
this utility expression, similar to when we parse the data ourselves
from the gdb_objc_realized_classes struct.

Worst case this will crash if the list is getting edited, which won't do
any harm and we'll just try again later.

Differential revision: https://reviews.llvm.org/D104951