[FIR] Set EnhancedNullability attribute for former flexible types

In some cases we transform flexible type into non-flexible during
  enhancement, but don't add `@EnhancedNullability` attribute to them,
  which breaks consistency with K1 on IR level

This commit fixes it

^KT-65302

compiler/testData/codegen/bytecodeListing/nullabilityAnnotations/nullabilityAnnotationsOnDelegatedMembers.jvm_abi.txt

@@ -7,3 +7,9 @@ MODULE main
 						(x: kotlin/String)
 					K2
 						(x: @kotlin/jvm/internal/EnhancedNullability kotlin/String)
+			FUNCTION takeNullable(Ljava/lang/String;)V
+				Property: class.metadata.function.valueParameters
+					K1
+						(x: kotlin/String?)
+					K2
+						(x: @kotlin/jvm/internal/EnhancedNullability kotlin/String?)