Move common entity fields to a 'base' struct. (#4161)

I'd considered moving DeclParams uses over, but when handling qualified
names, there's a parse node instead of an instruction. I did try to
unify a couple other uses though, including adding MergeDefinition. I
expect `interface` will use a little more once it's more completely
implemented, but maybe I'm wrong about that.

toolchain/check/decl_name_stack.cpp

@@ -380,6 +380,9 @@ auto DeclNameStack::ResolveAsScope(const NameContext& name_context,
                                name.params_id);
 
   // Find the scope corresponding to the resolved instruction.
+  // TODO: When diagnosing qualifiers on names, print a diagnostic that talks
+  // about qualifiers instead of redeclarations. Maybe also rename
+  // CheckRedeclParamsMatch.
   CARBON_KIND_SWITCH(context_->insts().Get(name_context.resolved_inst_id)) {
     case CARBON_KIND(SemIR::ClassDecl class_decl): {
       const auto& class_info = context_->classes().Get(class_decl.class_id);
@@ -414,6 +417,7 @@ auto DeclNameStack::ResolveAsScope(const NameContext& name_context,
     case CARBON_KIND(SemIR::Namespace resolved_inst): {
       auto scope_id = resolved_inst.name_scope_id;
       auto& scope = context_->name_scopes().Get(scope_id);
+      // This is specifically for qualified name handling.
       if (!CheckRedeclParamsMatch(
               *context_, new_params,
               DeclParams(name_context.resolved_inst_id, Parse::NodeId::Invalid,