[FE] Move @PrivateForInline optIn into :core:compiler.common

This is needed to be able to use it in backend modules too

compiler/fir/tree/src/org/jetbrains/kotlin/fir/annotations.kt

@@ -36,9 +36,3 @@ annotation class PrivateSessionConstructor
 @RequiresOptIn
 annotation class SessionConfiguration
 
-/**
- * [PrivateForInline] used for cases when there is a var property with mutable set and corresponding
- *   inline function which mutates this var
- */
-@RequiresOptIn
-annotation class PrivateForInline

compiler/fir/tree/src/org/jetbrains/kotlin/fir/caches/NullableMap.kt

@@ -5,7 +5,7 @@
 
 package org.jetbrains.kotlin.fir.caches
 
-import org.jetbrains.kotlin.fir.PrivateForInline
+import org.jetbrains.kotlin.util.PrivateForInline
 
 /**
  * [Map] which allows store null values

compiler/fir/tree/src/org/jetbrains/kotlin/fir/resolve/ScopeSession.kt

@@ -5,7 +5,7 @@
 
 package org.jetbrains.kotlin.fir.resolve
 
-import org.jetbrains.kotlin.fir.PrivateForInline
+import org.jetbrains.kotlin.util.PrivateForInline
 
 class ScopeSession {
     private val scopes = hashMapOf<Any, HashMap<ScopeSessionKey<*, *>, Any>>()