Instead of having an UnwindTable own a single assembly profiler,
and sharing it with all of its FuncUnwinders, have each FuncUnwinder
create an AssemblyProfiler on demand as needed.  I was worried that
the cost of creating the llvm disassemblers would be high for this
approach but it's not supposed to be an expensive operation, and it
means we don't need to add locks around this section of code.
<rdar://problem/16992332>