FE: add reproducer for KT-54004

compiler/testData/diagnostics/testsWithStdLib/builderInference/unsafeAssignment.kt

@@ -0,0 +1,20 @@
+class Foo<T : Any> {
+    fun doSmthng(arg: T) {}
+    var a: T? = null
+}
+
+fun <T : Any> myBuilder(block: Foo<T>.() -> Unit) : Foo<T> = Foo<T>().apply(block)
+
+fun main() {
+    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>myBuilder<!> {
+        <!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>a<!> = "some string"
+    }
+
+    val x = 57
+    val value = myBuilder {
+        doSmthng("one ")
+        a = 57
+        a = x
+    }
+    println(value.a?.count { it in 'l' .. 'q' })
+}