Avoid symbolic witnesses for .Self in an impl decl (#7564)

Point symbolic witnesses into `.Self` written inside an impl decl at the
impl that is being declared. This is tricky because the impl does not
yet exist. So we use a new instruction `ImplSelfWitness` which _will_ be
replaced by the `ImplWitness` once it becomes available. The
`ImplSelfWitness` acts like a symbolic witness, except it does not
perform lookup, since we know which impl we will get a witness from.

This prevents us from finding other impls when performing lookups into
`.Self` in an impl decl, which produces incorrect/incoherent results.

toolchain/check/context.cpp

@@ -74,6 +74,7 @@ auto Context::VerifyOnFinish() const -> void {
   vtable_stack_.VerifyOnFinish();
   region_stack_.VerifyOnFinish();
   CARBON_CHECK(impl_lookup_stack_.empty());
+  CARBON_CHECK(declaring_impl_decls_.empty());
   CARBON_CHECK(return_form_expr_ == std::nullopt);
 
 #ifndef NDEBUG