Add a no-op builtin function which shouldn't generate code. (#5306)

This is part of a broader plan to have noop destructor functions for
trivial destruction.

Note this emits a SemIR call (`%no_op: init %empty_tuple.type = call
%NoOp.ref() [concrete = constants.%empty_tuple]`), but not LLVM IR. My
thought was this was probably okay, since even though it'll be a little
spammy with destructor calls, the flipside is there'll probably already
be a fair amount for the name reference, and this at least shows when
the call is injected (and discarded).

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/check/eval.cpp

@@ -1379,6 +1379,16 @@ static auto MakeConstantForBuiltinCall(EvalContext& eval_context,
     case SemIR::BuiltinFunctionKind::None:
       CARBON_FATAL("Not a builtin function.");
 
+    case SemIR::BuiltinFunctionKind::NoOp: {
+      // Return an empty tuple value.
+      auto type_id = GetTupleType(eval_context.context(), {});
+      return MakeConstantResult(
+          eval_context.context(),
+          SemIR::TupleValue{.type_id = type_id,
+                            .elements_id = SemIR::InstBlockId::Empty},
+          phase);
+    }
+
     case SemIR::BuiltinFunctionKind::PrintChar:
     case SemIR::BuiltinFunctionKind::PrintInt:
     case SemIR::BuiltinFunctionKind::ReadChar: {