[FIR] Fix synthetic property is not var due to Nullable on parameter

Synthetic property is var when it have setter. The latter is set up
in property when its parameter type is equal to getter return type. In
case of using @Nullable, parameter type of setter is not equal to
return type of getter, because the latter is flexible type. So to fix
this verification should occur using not null types

#KT-39076 Fixed

compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/kt39076.kt

@@ -0,0 +1,13 @@
+// FILE: JavaClass.java
+import org.jetbrains.annotations.Nullable;
+
+public class JavaClass {
+    private String myFoo = "";
+    public String getFoo() { return myFoo; }
+    public void setFoo(@Nullable String s) { myFoo = s; }
+}
+
+// FILE: main.kt
+fun main(j: JavaClass) {
+    j.foo += "OK"
+}

compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/kt39076.txt

@@ -0,0 +1,4 @@
+FILE: main.kt
+    public final fun main(j: R|JavaClass|): R|kotlin/Unit| {
+        R|<local>/j|.R|/JavaClass.foo| = R|<local>/j|.R|/JavaClass.foo|.R|kotlin/String.plus|(String(OK))
+    }

compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithStdlibTestGenerated.java

@@ -870,6 +870,11 @@ public class FirDiagnosticsWithStdlibTestGenerated extends AbstractFirDiagnostic
             runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/KotlinClassParameterGeneric.kt");
         }
 
+        @TestMetadata("kt39076.kt")
+        public void testKt39076() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/kt39076.kt");
+        }
+
         @TestMetadata("LoggerInstance.kt")
         public void testLoggerInstance() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/LoggerInstance.kt");

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

@@ -18,8 +18,7 @@ import org.jetbrains.kotlin.fir.symbols.CallableId
 import org.jetbrains.kotlin.fir.symbols.StandardClassIds
 import org.jetbrains.kotlin.fir.symbols.SyntheticSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.*
-import org.jetbrains.kotlin.fir.types.ConeClassLikeType
-import org.jetbrains.kotlin.fir.types.FirResolvedTypeRef
+import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.load.java.propertyNameByGetMethodName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.util.capitalizeDecapitalize.capitalizeAsciiOnly
@@ -102,7 +101,7 @@ class FirSyntheticPropertiesScope(
                 val parameter = setter.valueParameters.singleOrNull() ?: return
                 if (setter.typeParameters.isNotEmpty() || setter.isStatic) return
                 val parameterType = (parameter.returnTypeRef as? FirResolvedTypeRef)?.type ?: return
-                if (parameterType != getterReturnType) return
+                if (getterReturnType.withNullability(ConeNullability.NOT_NULL) != parameterType.withNullability(ConeNullability.NOT_NULL)) return
                 matchingSetter = setter
             })
         }

compiler/testData/codegen/box/properties/javaPropertyBoxedGetter.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM
 
 // FILE: JavaClass.java

compiler/testData/codegen/box/properties/javaPropertyBoxedSetter.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM
 
 // FILE: JavaClass.java