[IR] Simplify IrOverridingUtil

Previously, it had some internal state that was saved between calls
and needed to be cleared.

This state was eliminated, which makes invariants clearer and using
easier.

^KT-61934

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/overrides/FakeOverrides.kt

@@ -281,7 +281,6 @@ class FakeOverrideBuilder(
 
     fun provideFakeOverrides(klass: IrClass, compatibilityMode: CompatibilityMode) {
         buildFakeOverrideChainsForClass(klass, compatibilityMode)
-        irOverridingUtil.clear()
         haveFakeOverrides.add(klass)
     }