Implement 'alias' with transparent semantics. (#3701)

Adds `BindAlias` with a hybrid of `BindName` and `NameRef` semantics. I
think it's slightly closer to `BindName` because it introduces a name,
so I'm going more in that direction. This also matches the need for
`bind_name_id` with imports on enclosing scopes.

Note, only things that look like a name reference are being allowed on
the RHS of `alias`. This includes builtins that look like name
references, such as `bool`, but not ones that turn into values
underneath, such as `false`.

toolchain/parse/handle_statement.cpp

@@ -10,6 +10,11 @@ auto HandleStatement(Context& context) -> void {
   context.PopAndDiscardState();
 
   switch (context.PositionKind()) {
+    case Lex::TokenKind::Alias: {
+      context.PushState(State::Alias);
+      context.AddLeafNode(NodeKind::AliasIntroducer, context.Consume());
+      break;
+    }
     case Lex::TokenKind::Break: {
       context.PushState(State::StatementBreakFinish);
       context.AddLeafNode(NodeKind::BreakStatementStart, context.Consume());