Clean up singleton TypeId use (#6300)

#6289 absentmindedly added fields in more places, and this is undoing
that plus further fixes.

This does some cleanup of types with relation to singletons. For
`TypeType` and `ErrorInst`, they're always complete due to a
`SetComplete` call in `file.cpp`. For `CppVoidType`, it's intended to be
incomplete by construction, and so a `TypeId` should be okay. The intent
though on not generally providing these had been that `GetSingletonType`
needs to be called to get a type to be marked as complete.

In the case of `AutoType`, removing `TypeId`does change a small printing
detail. I think that's old legacy that's just been carried forward.

Otherwise, for both `InstType` and `AutoType`, I've added
`GetSingletonType` calls where they were used in order to ensure
completeness is applied correctly. These calls cause small SemIR
permutations.

This causes `AutoType` to be seen by lowering, so I'm adding a
placeholder for it. Also merging two functions that look like they're
identical in intent -- not sure why they're separate.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/check/handle_binding_pattern.cpp

@@ -412,7 +412,8 @@ auto HandleParseNode(Context& context, Parse::AddrId node_id) -> bool {
     if (pointer_type) {
       auto addr_pattern_id = AddPatternInst<SemIR::AddrPattern>(
           context, node_id,
-          {.type_id = GetPatternType(context, SemIR::AutoType::TypeId),
+          {.type_id = GetPatternType(
+               context, GetSingletonType(context, SemIR::AutoType::TypeInstId)),
            .inner_id = param_pattern_id});
       context.node_stack().Push(node_id, addr_pattern_id);
     } else {