[FIR] Handle 'EnhancedNullability' more properly

This commit includes three changes:
1. 'EnhancedNullability' is no more set for declaration types
2. It is no more used for conversion types in translator
3. Translator inserts implicit not-null cast only when enhanced type is cast to not-null type.

compiler/testData/codegen/box/javaInterop/notNullAssertions/enhancedNullability/inLambdaReturnWithExpectedType.kt

@@ -1,5 +1,4 @@
 // !LANGUAGE: +StrictJavaNullabilityAssertions
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM
 
 // FILE: inLambdaReturnWithExpectedType.kt

compiler/testData/codegen/box/javaInterop/notNullAssertions/nullCheckOnLambdaReturnValue/nnStringVsT.kt

@@ -1,5 +1,4 @@
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
 // FILE: nnStringVsT.kt
 fun <T> useT(fn: () -> T) = fn()
 

compiler/testData/codegen/box/javaInterop/notNullAssertions/nullCheckOnLambdaReturnValue/nnStringVsTAny.kt

@@ -1,5 +1,4 @@
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
 // FILE: nnStringVsTAny.kt
 fun <T : Any> useTAny(fn: () -> T) = fn()