[FIR] Downgrade CanBeReplacedWithOperatorAssignmentChecker to experimental

This code wasn't considered trusted to be free from false positives
during the review.

^KT-69938

Author: lunakoly

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/ExperimentalExpressionCheckers.kt

@@ -23,4 +23,9 @@ object ExperimentalExpressionCheckers : ExpressionCheckers() {
         get() = setOf(
             RedundantInterpolationPrefixCheckerLiteral,
         )
+
+    override val variableAssignmentCheckers: Set<FirVariableAssignmentChecker>
+        get() = setOf(
+            CanBeReplacedWithOperatorAssignmentChecker,
+        )
 }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/ExtraExpressionCheckers.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.analysis.checkers
 
+import org.jetbrains.kotlin.fir.analysis.checkers.experimental.CanBeReplacedWithOperatorAssignmentChecker
 import org.jetbrains.kotlin.fir.analysis.checkers.expression.*
 import org.jetbrains.kotlin.fir.analysis.checkers.extra.*
 
@@ -14,11 +15,6 @@ object ExtraExpressionCheckers : ExpressionCheckers() {
             ArrayEqualityCanBeReplacedWithEquals,
         )
 
-    override val variableAssignmentCheckers: Set<FirVariableAssignmentChecker>
-        get() = setOf(
-            CanBeReplacedWithOperatorAssignmentChecker,
-        )
-
     override val qualifiedAccessExpressionCheckers: Set<FirQualifiedAccessExpressionChecker>
         get() = setOf(
             RedundantCallOfConversionMethod,

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/extra/CanBeReplacedWithOperatorAssignmentChecker.kt renamed to compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/experimental/CanBeReplacedWithOperatorAssignmentChecker.kt

@@ -1,9 +1,9 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2024 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
-package org.jetbrains.kotlin.fir.analysis.checkers.extra
+package org.jetbrains.kotlin.fir.analysis.checkers.experimental
 
 import org.jetbrains.kotlin.KtFakeSourceElementKind
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter