Refactor declaration name context logic to its own class. (#2989)

This started with cleaning up the remaining Name/expression type punning
in the node stack, and grew. I'm factoring out a class because we've
previously expressed the desire to factor logic out of SemanticsContext
where possible, and this seemed like a reasonable cut.

NameExpression as the first node as a QualifiedExpression allows the
qualifier handling to consider Name in one less spot, an incremental
simplification. However, the additional complexity caused by this makes
me split ApplyNameQualifier/ApplyExpressionQualifier in order to avoid
repeat checks of the parse node's kind. The logic is still largely
shared, thus a couple helper functions. I think this is all fairly well
structured in the isolated class.

I can see that we may want to avoid passing SemanticsContext as an
argument in the future if it elides a step of lookup.

toolchain/semantics/semantics_handle_namespace.cpp

@@ -9,16 +9,16 @@ namespace Carbon {
 
 auto SemanticsHandleNamespaceStart(SemanticsContext& context,
                                    ParseTree::Node /*parse_node*/) -> bool {
-  context.PushDeclarationName();
+  context.declaration_name_stack().Push();
   return true;
 }
 
 auto SemanticsHandleNamespace(SemanticsContext& context,
                               ParseTree::Node parse_node) -> bool {
-  auto name_context = context.PopDeclarationName();
+  auto name_context = context.declaration_name_stack().Pop();
   auto namespace_id = context.AddNode(SemanticsNode::Namespace::Make(
       parse_node, context.semantics_ir().AddNameScope()));
-  context.AddNameToLookup(name_context, namespace_id);
+  context.declaration_name_stack().AddNameToLookup(name_context, namespace_id);
   return true;
 }