Remove some designated initializers

With the switch to C++20, the rules became more strict, and we can no
longer initialize base classes. Make them comments instead.

(BTW
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2287r1.html
this offers some new syntax for this use-case. Hopefully this will be
adopted and we can eventually use it.)

src/libstore/content-address.cc

@@ -167,13 +167,13 @@ ContentAddressWithReferences caWithoutRefs(const ContentAddress & ca) {
         [&](const TextHash & h) -> ContentAddressWithReferences {
             return TextInfo {
                 h,
-                .references = {},
+                /* .references = */ {},
             };
         },
         [&](const FixedOutputHash & h) -> ContentAddressWithReferences {
             return FixedOutputInfo {
                 h,
-                .references = {},
+                /* .references = */ {},
             };
         },
     }, ca);