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_function.cpp

@@ -21,7 +21,7 @@ static auto BuildFunctionDeclaration(SemanticsContext& context)
   }
   SemanticsNodeBlockId param_refs_id =
       context.node_stack().Pop<ParseNodeKind::ParameterList>();
-  auto name_context = context.PopDeclarationName();
+  auto name_context = context.declaration_name_stack().Pop();
   auto fn_node = context.node_stack()
                      .PopForSoloParseNode<ParseNodeKind::FunctionIntroducer>();
 
@@ -36,7 +36,7 @@ static auto BuildFunctionDeclaration(SemanticsContext& context)
        .body_block_ids = {}});
   auto decl_id = context.AddNode(
       SemanticsNode::FunctionDeclaration::Make(fn_node, function_id));
-  context.AddNameToLookup(name_context, decl_id);
+  context.declaration_name_stack().AddNameToLookup(name_context, decl_id);
   return {function_id, decl_id};
 }
 
@@ -103,7 +103,7 @@ auto SemanticsHandleFunctionIntroducer(SemanticsContext& context,
   // Push the bracketing node.
   context.node_stack().Push(parse_node);
   // A name should always follow.
-  context.PushDeclarationName();
+  context.declaration_name_stack().Push();
   return true;
 }