Formalize Cpp as a PackageNameId (#6306)

This turns `Cpp` into a keyword, and makes it map to `NameId::Cpp` and
`PackageNameId::Cpp`.

Per discussion with zygoloid, the keyword versus identifier question is
deliberately kept open by #4846. This PR switches to a keyword because
mapping to a specific `PackageNameId` works best with a special `NameId`
not backed by an `IdentifierId`. We could in theory make it work using
`IdentifierId` or a runtime-tracked `PackageNameId` for `Cpp` (e.g.
stored on `SemIR::File`), but this approach is consistent with `Core`
and so seemed like a good starting point.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/parse/handle_expr.cpp

@@ -180,6 +180,11 @@ auto HandleExprInPostfix(Context& context) -> void {
       context.PushState(state);
       break;
     }
+    case Lex::TokenKind::Cpp: {
+      context.AddLeafNode(NodeKind::CppNameExpr, context.Consume());
+      context.PushState(state);
+      break;
+    }
     case Lex::TokenKind::SelfValueIdentifier: {
       context.AddLeafNode(NodeKind::SelfValueNameExpr, context.Consume());
       context.PushState(state);