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/parser/parser_handle_declaration_name_and_params.cpp

@@ -18,7 +18,10 @@ static auto ParserHandleDeclarationNameAndParams(ParserContext& context,
     context.PushState(state);
 
     if (context.PositionIs(TokenKind::Period)) {
-      context.AddLeafNode(ParseNodeKind::Name, *identifier);
+      // Because there's a qualifier, we process the first segment as an
+      // expression for simplicity. This just means semantics has one less thing
+      // to handle here.
+      context.AddLeafNode(ParseNodeKind::NameExpression, *identifier);
       state.state = ParserState::PeriodAsDeclaration;
       context.PushState(state);
     } else {