Use proper coroutine class descriptor for extension receiver generation

Before this commit context.getThisDescriptor() has been used, that was quite correct
until suspension happens inside inlined lambda (it has different this-descriptor)

compiler/testData/codegen/box/coroutines/suspendFromInlineLambda.kt

@@ -0,0 +1,31 @@
+class Controller {
+    suspend fun suspendHere(v: Int, x: Continuation<Int>) {
+        x.resume(v * 2)
+    }
+}
+
+fun builder(coroutine c: Controller.() -> Continuation<Unit>) {
+    c(Controller()).resume(Unit)
+}
+
+inline fun foo(x: (Int) -> Unit) {
+    for (i in 1..2) {
+        x(i)
+    }
+}
+
+fun box(): String {
+    var result = ""
+
+    builder {
+        result += "-"
+        foo {
+            result += suspendHere(it).toString()
+        }
+        result += "+"
+    }
+
+    if (result != "-24+") return "fail: $result"
+
+    return "OK"
+}