[FIR] Prevent StackOverflowError in FirTypeParameterSymbol

compiler/fir/tree/src/org/jetbrains/kotlin/fir/symbols/FirBasedSymbol.kt

@@ -27,6 +27,8 @@ abstract class FirBasedSymbol<E : FirDeclaration> {
         _fir = e
     }
 
+    val isBound get() = _fir != null
+
     val origin: FirDeclarationOrigin
         get() = fir.origin
 

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

@@ -22,7 +22,10 @@ class FirTypeParameterSymbol : FirClassifierSymbol<FirTypeParameter>() {
 
     override fun toLookupTag(): ConeTypeParameterLookupTag = lookupTag
 
-    override fun toString(): String = "${this::class.simpleName} ${name.asString()}"
+    override fun toString(): String = when {
+        isBound -> "${this::class.simpleName} ${name.asString()}"
+        else -> "${this::class.simpleName} <unbound>"
+    }
 
     val resolvedBounds: List<FirResolvedTypeRef>
         get() {