Rework handling of C++ references. (#6268)

For now, map C++ reference types to const-qualified Carbon pointer types
rather than picking between a (non-const) pointer or a value type. This
fixes misbehavior in lowering for reference members in classes and
reference return types.

Update the special-case handling for references as function parameters
so that it continues to map const reference parameters to Carbon
pass-by-value, and unify the code paths for `self` parameters and other
parameters, which were mostly doing the same thing but had some subtle
differences.

Add references to the list of types that we can pass to and from C++
directly, without needing an additional layer of thunks.

toolchain/check/pattern.cpp

@@ -157,15 +157,4 @@ auto AddSelfParamPattern(Context& context, SemIR::LocId loc_id,
   return pattern_id;
 }
 
-auto AddAddrSelfParamPattern(Context& context, SemIR::LocId loc_id,
-                             SemIR::ExprRegionId type_expr_region_id,
-                             SemIR::TypeInstId type_inst_id) -> SemIR::InstId {
-  auto pattern_id = AddSelfParamPattern(context, loc_id, type_expr_region_id,
-                                        GetPointerType(context, type_inst_id));
-  return AddPatternInst<SemIR::AddrPattern>(
-      context, loc_id,
-      {.type_id = GetPatternType(context, SemIR::AutoType::TypeId),
-       .inner_id = pattern_id});
-}
-
 }  // namespace Carbon::Check