[FIR] Do not allow vararg function to be used as synthetic setter

^KT-63172 Fixed

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/calls/Synthetics.kt

@@ -133,8 +133,9 @@ class FirSyntheticPropertiesScope private constructor(
                 if (matchingSetter != null) return
 
                 val setter = setterSymbol.fir
-                val parameter = setter.valueParameters.singleOrNull() ?: return
                 if (setter.typeParameters.isNotEmpty() || setter.isStatic) return
+                val parameter = setter.valueParameters.singleOrNull() ?: return
+                if (parameter.isVararg) return
                 val parameterType = (parameter.returnTypeRef as? FirResolvedTypeRef)?.type ?: return
                 if (!setterTypeIsConsistentWithGetterType(propertyName, getterSymbol, setterSymbol, parameterType)) return
                 matchingSetter = setterSymbol.fir

compiler/testData/diagnostics/tests/syntheticExtensions/javaProperties/FalseSetters.fir.kt

@@ -4,7 +4,7 @@ fun foo(javaClass: JavaClass) {
     javaClass.<!VAL_REASSIGNMENT!>something2<!>++
     javaClass.<!VAL_REASSIGNMENT!>something3<!>++
     javaClass.<!VAL_REASSIGNMENT!>something4<!>++
-    javaClass.something5 = null
+    javaClass.<!VAL_REASSIGNMENT!>something5<!> = null
 }
 
 // FILE: JavaClass.java