FIR checkers: report VAL_REASSIGNMENT for assignment operators

Currently VAL_REASSIGNMENT are only reported on direct assignments.
Reassignments in the form of, for example, `+=` are reported as
`VARIABLE_EXPECTED`, which differs from FE1.0.

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/diagnostics/FirDiagnostics.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.fir.symbols.AbstractFirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirCallableSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassLikeSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirTypeParameterSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirVariableSymbol
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
@@ -59,6 +60,10 @@ class ConeVariableExpectedError : ConeDiagnostic() {
     override val reason: String get() = "Variable expected"
 }
 
+class ConeValReassignmentError(val variable: FirVariableSymbol<*>) : ConeDiagnostic() {
+    override val reason: String get() = "Re-assigning a val variable"
+}
+
 class ConeTypeMismatchError(val expectedType: ConeKotlinType, val actualType: ConeKotlinType) : ConeDiagnostic() {
     override val reason: String
         get() = "Type mismatch. Expected: $expectedType, Actual: $actualType"

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt

@@ -385,7 +385,8 @@ open class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransform
         val operatorIsError = operatorCallReference?.isError ?: true
 
         val lhsReference = leftArgument.toResolvedCallableReference()
-        val lhsVariable = (lhsReference?.resolvedSymbol as? FirVariableSymbol<*>)?.fir
+        val lhsSymbol = lhsReference?.resolvedSymbol as? FirVariableSymbol<*>
+        val lhsVariable = lhsSymbol?.fir
         val lhsIsVar = lhsVariable?.isVar == true
         return when {
             operatorIsError || (!lhsIsVar && !assignIsError) -> {
@@ -409,7 +410,7 @@ open class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransform
                                         leftArgument.calleeReference.source
                                     else -> leftArgument.source
                                 }
-                                diagnostic = ConeVariableExpectedError()
+                                diagnostic = if (lhsSymbol == null) ConeVariableExpectedError() else ConeValReassignmentError(lhsSymbol)
                             }
                         }
                         (leftArgument as? FirQualifiedAccess)?.let {