Avoid ToTokenOnly for the AddrSelfIsNonRef diagnostic (#5371)

The location of the diagnostic is an instruction id, not a parse node.
Converting to a parse node to call ToTokenOnly will drop the descendents
of the instruction being diagnosed, but the diagnostic is about the
whole instruction not just the root parse node of whatever instruction
it happens to be.

toolchain/check/pattern_match.cpp

@@ -261,9 +261,7 @@ auto MatchContext::DoEmitPatternMatch(Context& context,
     default:
       CARBON_DIAGNOSTIC(AddrSelfIsNonRef, Error,
                         "`addr self` method cannot be invoked on a value");
-      context.emitter().Emit(
-          context.insts().GetCanonicalLocId(entry.scrutinee_id).ToTokenOnly(),
-          AddrSelfIsNonRef);
+      context.emitter().Emit(entry.scrutinee_id, AddrSelfIsNonRef);
       // Add fake reference expression to preserve invariants.
       auto scrutinee = context.insts().GetWithLocId(entry.scrutinee_id);
       scrutinee_ref_id = AddInstWithCleanup<SemIR::TemporaryStorage>(