[PSI, FE, PSI2IR] Use labels for referencing specific receiver

compiler/testData/diagnostics/tests/extensions/contextReceivers/thisWithCustomLabel.kt

@@ -0,0 +1,22 @@
+class A<T>(val a: T)
+class B(val b: Any)
+class C(val c: Any)
+
+context(labelAInt@A<Int>, A<String>, labelB@B) fun f() {
+    this@labelAInt.a.toFloat()
+    this@A.a.length
+    this@labelB.b
+    this<!UNRESOLVED_REFERENCE!>@B<!>
+}
+
+context(labelAInt@A<Int>, A<String>, labelB@B) val C.p: Int
+    get() {
+        this@labelAInt.a.toFloat()
+        this@A.a.length
+        this<!UNRESOLVED_REFERENCE!>@B<!>
+        this@labelB.b
+        this@C.c
+        this@p.c
+        this.c
+        return 1
+    }