[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/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