[IC] Add back flush() API to PersistentStorage

In commit 4e89dcf, we removed `flush()` from `PersistentStorage`
(previously called `LazyStorage`) because it is not used in Gradle
builds.

However, `flush()` is still used in JPS builds, so we need to add that
API back in this commit.

#KT-62486 Fixed

Co-authored-by: Hung Nguyen <hungnv@google.com>


Merge-request: KOTLIN-MR-796
Merged-by: Aleksei Cherepanov <aleksei.cherepanov@jetbrains.com>

build-common/test/org/jetbrains/kotlin/incremental/CompilationTransactionTest.kt

@@ -39,6 +39,8 @@ private class InMemoryStorageWrapperMock : InMemoryStorageInterface<Any, Any> {
 
     override val keys: Set<Any> = emptySet()
 
+    override fun flush() {}
+
     override fun close() {}
 
     override fun remove(key: Any) {}