[FIR] Refactor: extract function which returns single compatible expect

...symbol for actual symbol. It will be reused in next commit.

^KT-62036

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExpectActualDeclarationChecker.kt

@@ -188,7 +188,7 @@ object FirExpectActualDeclarationChecker : FirBasicDeclarationChecker() {
             else -> {}
         }
         // We want to report errors even if a candidate is incompatible, but it's single
-        val expectedSingleCandidate = compatibilityToMembersMap[Compatible]?.singleOrNull()
+        val expectedSingleCandidate = symbol.getSingleCompatibleExpectForActualOrNull()
             ?: symbol.getSingleExpectForActualOrNull()
         if (expectedSingleCandidate != null) {
             checkIfExpectHasDefaultArgumentsAndActualizedWithTypealias(

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/ExpectActualAttributes.kt

@@ -27,6 +27,10 @@ fun FirBasedSymbol<*>.getSingleExpectForActualOrNull(): FirBasedSymbol<*>? {
     return expectForActual?.values?.singleOrNull()?.singleOrNull()
 }
 
+fun FirBasedSymbol<*>.getSingleCompatibleExpectForActualOrNull(): FirBasedSymbol<*>? {
+    return expectForActual?.get(ExpectActualCompatibility.Compatible)?.singleOrNull()
+}
+
 fun FirBasedSymbol<*>.getSingleCompatibleOrWeaklyIncompatibleExpectForActualOrNull(): FirBasedSymbol<*>? {
     val expectForActual = expectForActual ?: return null
     val compatibleOrWeakCompatible: List<FirBasedSymbol<*>> =