Move the FacetAccessType special case out of name lookup, and generalize it (#6163)

The `AppendLookupScopesForConstant` function had a special case for
`facet as type` which was overly broad (applying to all callers to the
function when only one caller needs it), and was confusingly overly
specific (applying to `facet as type` but not to `facet` constants).

We clarify all of this by moving it out to member access, and applying
it only to the case of looking into the type of `base_id`. In that case
we are doing member lookup into the facet itself, but since it's
symbolic we don't know the type to look into. And we don't defer the
lookup with a symbolic instruction, so we do the lookup into the facet's
type instead.

We add a helper function in member access, `ExtractFacetTypeForFacet` to
encapsulate this slightly-odd operation. It's odd because it ends up
getting *the type of the type* when the `base_id` has a facet as its
type.

The helper is now built on top of GetCanonicalFacetOrTypeValue() instead
of explicitly looking for FacetAccessType, which makes it work more
generally for any type instructions that represent a facet, including
SymbolicBindingType in the future.

While here document and improve clarity throughout the
`PerformActionHelper` for member access.

Author: danakj

toolchain/check/impl_lookup.cpp

@@ -882,6 +882,9 @@ auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
     }
   }
 
+  // Ensure specifics don't substitute in weird things for the query self.
+  CARBON_CHECK(context.types().IsFacetType(
+      context.insts().Get(eval_query.query_self_inst_id).type_id()));
   SemIR::ConstantId query_self_const_id =
       context.constant_values().Get(eval_query.query_self_inst_id);