IR: optimize IrDeclarationWithName.fqNameWhenAvailable

Do not construct lots of FqName objects, since doing so takes ~1% of JVM
backend time.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrUtils.kt

@@ -316,13 +316,29 @@ val IrClass.isClass get() = kind == ClassKind.CLASS
 val IrClass.isObject get() = kind == ClassKind.OBJECT
 val IrClass.isAnonymousObject get() = isClass && name == SpecialNames.NO_NAME_PROVIDED
 val IrClass.isNonCompanionObject: Boolean get() = isObject && !isCompanion
+
 val IrDeclarationWithName.fqNameWhenAvailable: FqName?
-    get() = when (val parent = parent) {
-        is IrDeclarationWithName -> parent.fqNameWhenAvailable?.child(name)
-        is IrPackageFragment -> parent.packageFqName.child(name)
-        else -> null
+    get() {
+        val sb = StringBuilder()
+        return if (computeFqNameString(this, sb)) FqName(sb.toString()) else null
     }
 
+private fun computeFqNameString(declaration: IrDeclarationWithName, result: StringBuilder): Boolean {
+    when (val parent = declaration.parent) {
+        is IrDeclarationWithName -> {
+            if (!computeFqNameString(parent, result)) return false
+        }
+        is IrPackageFragment -> {
+            val packageFqName = parent.packageFqName
+            if (!packageFqName.isRoot) result.append(packageFqName)
+        }
+        else -> return false
+    }
+    if (result.isNotEmpty()) result.append('.')
+    result.append(declaration.name.asString())
+    return true
+}
+
 val IrDeclaration.parentAsClass: IrClass
     get() = parent as? IrClass
         ?: error("Parent of this declaration is not a class: ${render()}")