this is a forward port of scala/scala#10397 (fixing scala/bug#12783, from which Scala 3 also suffers) — the same fix we'll be shipping in 2.12.18 and 2.13.11

I haven't included a unit test, because

• a similar change has already been well validated in a Scala 2 context
• without this change, the compiler can't compile anything at all on JDK 21
• we need the reference compiler to include this change before we can bootstrap on JDK 21 anyway

but I did manually test it locally, by bootstrapping on JDK 11, publishing the resulting compiler locally, and then launching the REPL on JDK 21 and evaluating 2 + 2

on 3.3.0-RC6, that fails with:

error while loading AccessFlag,
class file /modules/java.base/java/lang/reflect/AccessFlag.class is broken, reading aborted with class java.lang.RuntimeException
bad constant pool index: 0 at pos: 5189
error while loading ElementType,
class file /modules/java.base/java/lang/annotation/ElementType.class is broken, reading aborted with class java.lang.RuntimeException
bad constant pool index: 0 at pos: 1220
2 errors found

whereas with this change, it succeeds.