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/function.cpp

@@ -73,7 +73,7 @@ auto CheckFunctionTypeMatches(Context& context,
 }
 
 auto CheckFunctionReturnType(Context& context, SemIR::LocId loc_id,
-                             SemIR::Function& function,
+                             const SemIR::Function& function,
                              SemIR::SpecificId specific_id)
     -> SemIR::ReturnTypeInfo {
   auto return_info = SemIR::ReturnTypeInfo::ForFunction(context.sem_ir(),