ICE in NLL borrow_check/universal_regions when bounding a trait object with nothing

[Centril]

Found as a byproduct of work on #61311.

struct A {}
trait T {}
impl A where dyn T: {
    fn f() {}
}

backtrace:

error: internal compiler error: src/librustc_mir/borrow_check/nll/universal_regions.rs:741: cannot convert `ReEmpty` to a region vid

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:637:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::bug
   8: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::bug_fmt
  14: rustc_mir::borrow_check::nll::universal_regions::UniversalRegionIndices::to_region_vid::{{closure}}
  15: rustc_mir::borrow_check::nll::type_check::free_region_relations::create
  16: rustc_mir::borrow_check::nll::type_check::type_check
  17: rustc_mir::borrow_check::nll::compute_regions
  18: rustc_mir::borrow_check::do_mir_borrowck
  19: rustc::ty::context::GlobalCtxt::enter_local
  20: rustc_mir::borrow_check::mir_borrowck
  21: rustc::ty::query::__query_compute::mir_borrowck
  22: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
  23: rustc::dep_graph::graph::DepGraph::with_task_impl
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  25: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  26: rustc::util::common::time
  27: rustc_interface::passes::analysis
  28: rustc::ty::query::__query_compute::analysis
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: rustc::ty::context::tls::enter_global
  33: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  34: rustc_interface::passes::create_global_ctxt::{{closure}}
  35: rustc_interface::interface::run_compiler_in_existing_thread_pool
  36: std::thread::local::LocalKey<T>::with
  37: scoped_tls::ScopedKey<T>::set
  38: syntax::with_globals
query stack during panic:
#0 [mir_borrowck] processing `A::f`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.36.0-beta.1 (3a5d62bd5 2019-05-22) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

Compiles on stable 1.35.0.

[pnkfelix]

triage: Looks like matthewjasper is on top of this. Marking P-high just so I don't have to think about it further right now. Removing nomination tag.