Skip assert ICE with default_method_body_is_const

functions marked with #[default_method_body_is_const] would
ICE when being const checked due to it not being a const function:
`tcx.is_const_fn_raw(did)` returns false. We should skip this assert
when it is marked with that attribute.

compiler/rustc_mir/src/transform/check_consts/mod.rs

@@ -9,7 +9,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::mir;
 use rustc_middle::ty::{self, TyCtxt};
-use rustc_span::Symbol;
+use rustc_span::{sym, Symbol};
 
 pub use self::qualifs::Qualif;
 
@@ -104,6 +104,13 @@ pub fn rustc_allow_const_fn_unstable(
 pub fn is_const_stable_const_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
     use attr::{ConstStability, Stability, StabilityLevel};
 
+    // A default body marked const is not const-stable because const
+    // trait fns currently cannot be const-stable. We shouldn't
+    // restrict default bodies to only call const-stable functions.
+    if tcx.has_attr(def_id, sym::default_method_body_is_const) {
+        return false;
+    }
+
     // Const-stability is only relevant for `const fn`.
     assert!(tcx.is_const_fn_raw(def_id));