K2: Fix compilation after making Candidate::symbol a var

See the previous commits

Explicit cast became necessary as the smart cast stopped working there

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CallableReferenceResolution.kt

@@ -48,7 +48,7 @@ internal object CheckCallableReferenceExpectedType : CheckerStage() {
             else -> null
         }
 
-        val fir: FirCallableDeclaration = candidate.symbol.fir
+        val fir: FirCallableDeclaration = candidate.symbol.fir as FirCallableDeclaration
 
         val (rawResultingType, callableReferenceAdaptation) = buildReflectionType(fir, resultingReceiverType, candidate, context)
         val resultingType = candidate.substitutor.substituteOrSelf(rawResultingType)

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeOverloadConflictResolver.kt

@@ -117,9 +117,10 @@ class ConeOverloadConflictResolver(
     }
 
     private fun Candidate.overrides(other: Candidate): Boolean {
+        val symbol = symbol
         if (symbol !is FirCallableSymbol || other.symbol !is FirCallableSymbol) return false
 
-        val otherOriginal = other.symbol.unwrapSubstitutionOverrides()
+        val otherOriginal = (other.symbol as FirCallableSymbol).unwrapSubstitutionOverrides()
         if (symbol.unwrapSubstitutionOverrides<FirCallableSymbol<*>>() == otherOriginal) return true
 
         val scope = originScope as? FirTypeScope ?: return false