[lldb] update `monobt` so it understands interpreter frames.

[lldb] update `monobt` so it understands interpreter frames.

```
(lldb) monobt
* thread #1
  * frame #0: 0x00000001001b236e mono-sgen`interp_transform_call(td=0x00007fff5fbfd080, method=0x0000000100915a90, target_method=0x0000000000000000, domain=0x000000010090b741
    frame #1: 0x00000001001a1c2e mono-sgen`generate(method=0x0000000100915a90, rtm=0x000000010382ac70, is_bb_start="\x01", generic_context=0x0000000100915ad0) + 9454 at tran8
    transforming TestMonoAsyncGenerics::AsyncWithAwait || frame #2: 0x000000010019f553 mono-sgen`mono_interp_transform_method(runtime_method=0x000000010382ac70, context=0x004
    TestMonoAsyncGenerics::AsyncWithAwait @ 0  || frame #3: 0x000000010018a178 mono-sgen`ves_exec_method_with_context(frame=0x00007fff5fbfe290, context=0x00007fff5fbfe3a8) +9
    TestMonoAsyncGenerics::Main @ 12 "pop" || frame #4: 0x000000010018b4b1 mono-sgen`ves_exec_method_with_context(frame=0x00007fff5fbfe420, context=0x00007fff5fbfe3a8) + 5081
    frame #5: 0x0000000100189e43 mono-sgen`mono_interp_runtime_invoke(method=0x000000010090ce38, obj=0x0000000000000000, params=0x00007fff5fbfea40, exc=0x0000000000000000, e0
    frame #6: 0x00000001000164a2 mono-sgen`mono_jit_runtime_invoke(method=0x000000010090ce38, obj=0x0000000000000000, params=0x00007fff5fbfea40, exc=0x0000000000000000, erro1
    frame #7: 0x000000010038b2b5 mono-sgen`do_runtime_invoke(method=0x000000010090ce38, obj=0x0000000000000000, params=0x00007fff5fbfea40, exc=0x0000000000000000, error=0x002
    frame #8: 0x0000000100384e97 mono-sgen`mono_runtime_invoke_checked(method=0x000000010090ce38, obj=0x0000000000000000, params=0x00007fff5fbfea40, error=0x00007fff5fbfeb000
    frame #9: 0x000000010038f335 mono-sgen`do_exec_main_checked(method=0x000000010090ce38, args=0x00000001020003c8, error=0x00007fff5fbfeb00) + 197 at object.c:4672
    frame #10: 0x000000010038dd5c mono-sgen`mono_runtime_exec_main_checked(method=0x000000010090ce38, args=0x00000001020003c8, error=0x00007fff5fbfeb00) + 76 at object.c:4773
    frame #11: 0x000000010038ddbf mono-sgen`mono_runtime_run_main_checked(method=0x000000010090ce38, argc=1, argv=0x00007fff5fbfef68, error=0x00007fff5fbfeb00) + 79 at objec2
    frame #12: 0x00000001000d9a33 mono-sgen`mono_jit_exec(domain=0x000000010090b740, assembly=0x0000000100913610, argc=1, argv=0x00007fff5fbfef68) + 403 at driver.c:1029
    frame #13: 0x00000001000dd9da mono-sgen`main_thread_handler(user_data=0x00007fff5fbfeea0) + 538 at driver.c:1098
    frame #14: 0x00000001000dc21c mono-sgen`mono_main(argc=3, argv=0x00007fff5fbfef58) + 8636 at driver.c:2163
    frame #15: 0x0000000100001b9e mono-sgen`mono_main_with_options(argc=3, argv=0x00007fff5fbfef58) + 46 at main.c:45
    frame #16: 0x00000001000012dd mono-sgen`main(argc=3, argv=0x00007fff5fbfef58) + 77 at main.c:338
    frame #17: 0x00007fffc2e66255 libdyld.dylib`start + 1
    frame #18: 0x00007fffc2e66255 libdyld.dylib`start + 1
```