Validate before reporting interning errors.

validation produces much higher quality errors and already handles most of the cases

compiler/rustc_const_eval/src/interpret/mod.rs

@@ -23,6 +23,7 @@ pub use rustc_middle::mir::interpret::*; // have all the `interpret` symbols in
 pub use self::eval_context::{format_interp_error, Frame, FrameInfo, InterpCx, StackPopCleanup};
 pub use self::intern::{
     intern_const_alloc_for_constprop, intern_const_alloc_recursive, HasStaticRootDefId, InternKind,
+    InternResult,
 };
 pub use self::machine::{compile_time_machine, AllocMap, Machine, MayLeak, StackPopJump};
 pub use self::memory::{AllocKind, AllocRef, AllocRefMut, FnVal, Memory, MemoryKind};