Track the instruction used to name the type and constraint in an impl. (#4368)

This is necessary in order to have access to the specific versions of
their constant values in a generic impl.

Stub out impl deduction.

toolchain/check/node_stack.h

@@ -391,7 +391,7 @@ class NodeStack {
     set_id_if_category_is(Parse::NodeCategory::MemberName,
                           Id::KindFor<SemIR::NameId>());
     set_id_if_category_is(Parse::NodeCategory::ImplAs,
-                          Id::KindFor<SemIR::TypeId>());
+                          Id::KindFor<SemIR::InstId>());
     set_id_if_category_is(Parse::NodeCategory::Decl |
                               Parse::NodeCategory::Statement |
                               Parse::NodeCategory::Modifier,