Fix Scala 2 library TASTy synthetic unapply optimizations (#19023)

Issue: Synthetic unapplies from the Scala 2 library TASTy do not have
the `Case` flag.


Alternative (4f681a1): Set the `Case`
flag when compiling the Scala 2 library.

Author: nicolasstucki

Diff for: .github/workflows/ci.yaml

@@ -144,7 +144,7 @@ jobs:
           ./project/scripts/sbt ";sjsSandbox/run ;sjsSandbox/test ;sjsJUnitTests/test ;set sjsJUnitTests/scalaJSLinkerConfig ~= switchToESModules ;sjsJUnitTests/test ;sjsCompilerTests/test"
 
       - name: Test with Scala 2 library TASTy (fast)
-        run: ./project/scripts/sbt ";set ThisBuild/Build.useScala2LibraryTasty := true ;scala3-bootstrapped/testCompilation i5; scala3-bootstrapped/testCompilation tests/run/typelevel-peano.scala" # only test a subset of test to avoid doubling the CI execution time
+        run: ./project/scripts/sbt ";set ThisBuild/Build.useScala2LibraryTasty := true ;scala3-bootstrapped/testCompilation i5; scala3-bootstrapped/testCompilation tests/run/typelevel-peano.scala; scala3-bootstrapped/testOnly dotty.tools.backend.jvm.DottyBytecodeTests" # only test a subset of test to avoid doubling the CI execution time
 
   test_scala2_library_tasty:
     runs-on: [self-hosted, Linux]

Diff for: compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -330,7 +330,7 @@ object PatternMatcher {
         lazy val caseAccessors = caseClass.caseAccessors
 
         def isSyntheticScala2Unapply(sym: Symbol) =
-          sym.isAllOf(SyntheticCase) && sym.owner.is(Scala2x)
+          sym.is(Synthetic) && sym.owner.is(Scala2x)
 
         def tupleApp(i: Int, receiver: Tree) = // manually inlining the call to NonEmptyTuple#apply, because it's an inline method
           ref(defn.RuntimeTuplesModule)