[FIR] Introduce FirValueParameterSymbol.defaultValueSource

It's easier to read the code if you don't
see generic `fir` access and thus don't
need to check all its usages to make sure
it's used correctly.

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

@@ -221,15 +221,13 @@ object FirOverrideChecker : FirAbstractOverrideChecker() {
         }
     }
 
-    @OptIn(SymbolInternals::class)
     private fun FirFunctionSymbol<*>.checkDefaultValues(
         reporter: DiagnosticReporter,
         context: CheckerContext,
     ) {
         for (valueParameterSymbol in valueParameterSymbols) {
-            val defaultValue = valueParameterSymbol.fir.defaultValue
-            if (defaultValue != null) {
-                reporter.reportOn(defaultValue.source, FirErrors.DEFAULT_VALUE_NOT_ALLOWED_IN_OVERRIDE, context)
+            if (valueParameterSymbol.hasDefaultValue) {
+                reporter.reportOn(valueParameterSymbol.defaultValueSource, FirErrors.DEFAULT_VALUE_NOT_ALLOWED_IN_OVERRIDE, context)
             }
         }
     }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/symbols/impl/FirVariableSymbol.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.symbols.impl
 
+import org.jetbrains.kotlin.KtSourceElement
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.diagnostics.ConeDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnonymousObjectExpression
@@ -115,6 +116,9 @@ class FirValueParameterSymbol(name: Name) : FirVariableSymbol<FirValueParameter>
     val hasDefaultValue: Boolean
         get() = fir.defaultValue != null
 
+    val defaultValueSource: KtSourceElement?
+        get() = fir.defaultValue?.source
+
     val isCrossinline: Boolean
         get() = fir.isCrossinline