Initial 'export name' handling. (#3949)

This adds a `BindExport` instruction in order to better track the
location of the `export` itself, but a `bind_name_id` is also added to
`ImportRef` so that we know quickly where to put it in name lookup.

Merging identical names is a TODO. I haven't quite decided how best to
achieve that, because I do think the BindExport should be what's
actually added to name lookup.

Also, I will probably add a mode to DeclNameStack that blocks
non-namespace scopes. This seems to already be an error, but the wrong
one (maybe due to lack of support for cross-file decl/def support).

toolchain/check/impl.cpp

@@ -106,8 +106,8 @@ static auto BuildInterfaceWitness(
           CARBON_FATAL() << "Unexpected type: " << type_inst;
         }
         auto& fn = context.functions().Get(fn_type->function_id);
-        auto impl_decl_id =
-            context.LookupNameInExactScope(decl_id, fn.name_id, impl_scope);
+        auto impl_decl_id = context.LookupNameInExactScope(
+            decl_id, fn.name_id, impl.scope_id, impl_scope);
         if (impl_decl_id.is_valid()) {
           used_decl_ids.push_back(impl_decl_id);
           table.push_back(CheckAssociatedFunctionImplementation(