rustdoc ICE for intra links referring to builtin macros

[Manishearth]

/// [foo](format_args)
struct Foo;

when passed through rustdoc gives the following crash:

thread 'rustc' panicked at 'index out of bounds: the len is 11 but the index is 4294967295', /Users/travis/build/rust-lang/rust/src/liballoc/vec.rs:1551:10
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
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: std::panicking::begin_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic_bounds_check
  10: rustc::ty::maps::<impl rustc::ty::maps::queries::crate_name<'tcx>>::compute_result
  11: rustc::dep_graph::graph::DepGraph::with_task_impl
  12: rustc_errors::Handler::track_diagnostics
  13: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check
  14: rustc::ty::maps::<impl rustc::ty::maps::queries::crate_name<'tcx>>::force
  15: rustc::ty::maps::<impl rustc::ty::maps::queries::crate_name<'tcx>>::try_get
  16: rustc::ty::maps::TyCtxtAt::crate_name
  17: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::crate_name
  18: rustdoc::clean::inline::record_extern_fqn
  19: rustdoc::clean::register_def
  20: <[syntax::ast::Attribute] as rustdoc::clean::Clean<rustdoc::clean::Attributes>>::clean
  21: <rustdoc::doctree::Struct as rustdoc::clean::Clean<rustdoc::clean::Item>>::clean
  22: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::spec_extend
  23: <rustdoc::doctree::Module as rustdoc::clean::Clean<rustdoc::clean::Item>>::clean
  24: <rustdoc::visit_ast::RustdocVisitor<'a, 'tcx, 'rcx> as rustdoc::clean::Clean<rustdoc::clean::Crate>>::clean
  25: rustdoc::core::run_core::{{closure}}
  26: <std::thread::local::LocalKey<T>>::with
  27: <std::thread::local::LocalKey<T>>::with
  28: rustc::ty::context::TyCtxt::create_and_enter
  29: rustc_driver::driver::phase_3_run_analysis_passes
  30: rustdoc::core::run_core

error: internal compiler error: unexpected panic

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 1.25.0-nightly (3d292b793 2018-02-03) running on x86_64-apple-darwin

This is because resolve_macro returns DefIds containing CrateNum = -1 (cratenum is an unsigned integer, 🤷‍♀️) for syntax extensions

[Manishearth]

@jseyfried FWIW, the debug impl on DefIDs panics when you attempt to run it on BUILTIN_MACROS_CRATE defs.

(also other things panic since they expect the crate number to be normal)

[Manishearth]

This gets fixed in #48335