Add test for generic setValue called via synthetic accessor

Synthetic accessor for 'setValue' was generated incorrectly,
specific case of KT-20491 (Incorrect synthetic accessor generated for a
generic base class function specialized with primitive type).

Make sure there's no equivalent of KT-20387 for delegated properties.

compiler/testData/codegen/box/delegatedProperty/genericSetValueViaSyntheticAccessor.kt

@@ -0,0 +1,25 @@
+// FILE: Var.kt
+package pvar
+
+open class PVar<T>(private var value: T) {
+    protected operator fun getValue(thisRef: Any?, prop: Any?) = value
+
+    protected operator fun setValue(thisRef: Any?, prop: Any?, newValue: T) {
+        value = newValue
+    }
+}
+
+// FILE: test.kt
+import pvar.*
+
+class C : PVar<Long>(42L) {
+    inner class Inner {
+        var x by this@C
+    }
+}
+
+fun box(): String {
+    val inner = C().Inner()
+    inner.x = 1L
+    return "OK"
+}