[FIR2IR] Cache type parameters for existing IR classes properly

Before this commit, we just recreated class header
for external IR class found by signature. However, it's not required,
because super-types, type parameters, etc. are already initialized.
So now we just read type parameters and build cache for them.

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrClassifierStorage.kt

@@ -429,7 +429,10 @@ class Fir2IrClassifierStorage(
         symbolTable.referenceClassIfAny(signature)?.let { irClassSymbol ->
             val irClass = irClassSymbol.owner
             classCache[firClass as FirRegularClass] = irClass
-            processClassHeader(firClass, irClass)
+            val mappedTypeParameters = firClass.typeParameters.filterIsInstance<FirTypeParameter>().zip(irClass.typeParameters)
+            for ((firTypeParameter, irTypeParameter) in mappedTypeParameters) {
+                typeParameterCache[firTypeParameter] = irTypeParameter
+            }
             declarationStorage.preCacheBuiltinClassMembers(firClass, irClass)
             return irClassSymbol
         }