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_node_stack.h

@@ -82,7 +82,8 @@ class SemanticsNodeStack {
     PopForSoloParseNode<RequiredParseKind>();
   }
 
-  // Pops the top of the stack and returns the parse_node and the ID.
+  // Pops an expression from the top of the stack and returns the parse_node and
+  // the ID.
   auto PopExpressionWithParseNode()
       -> std::pair<ParseTree::Node, SemanticsNodeId> {
     return PopWithParseNode<SemanticsNodeId>();
@@ -325,12 +326,6 @@ class SemanticsNodeStack {
 
   // Require a ParseNodeKind be mapped to a particular IdKind.
   auto RequireIdKind(ParseNodeKind parse_kind, IdKind id_kind) -> void {
-    // TODO: Name can be popped as a node_id by declaration name handling. Will
-    // refactor to remove this quirk.
-    if (parse_kind == ParseNodeKind::Name &&
-        id_kind == IdKind::SemanticsNodeId) {
-      return;
-    }
     CARBON_CHECK(ParseNodeKindToIdKind(parse_kind) == id_kind)
         << "Unexpected IdKind mapping for " << parse_kind;
   }