Standardize context receiver parameter names

Previously, FIR used `_context_receiver_n` while FE10 used `<this>` for
all context receiver parameters. This commit changes the code in FE10
to follow the convention from FIR.

compiler/testData/ir/irText/declarations/contextReceivers/fromKEEP/monoidSum.kt.txt

@@ -43,20 +43,20 @@ object StringMonoid : Monoid<String> {
 
 }
 
-fun <T : Any?> List<T>.sum(<this>: Monoid<T>): T {
-  return <this>.fold<T, T>(initial = <this>.<get-unit>(), operation = local fun <anonymous>(acc: T, e: T): T {
-    return (<this>, acc).combine(other = e)
+fun <T : Any?> List<T>.sum(_context_receiver_0: Monoid<T>): T {
+  return <this>.fold<T, T>(initial = _context_receiver_0.<get-unit>(), operation = local fun <anonymous>(acc: T, e: T): T {
+    return (_context_receiver_0, acc).combine(other = e)
   }
 )
 }
 
 fun box(): String {
   with<IntMonoid, Int>(receiver = IntMonoid, block = local fun IntMonoid.<anonymous>(): Int {
-    return listOf<Int>(elements = [1, 2, 3]).sum<Int>(<this> = $this$with)
+    return listOf<Int>(elements = [1, 2, 3]).sum<Int>(_context_receiver_0 = $this$with)
   }
 ) /*~> Unit */
   return with<StringMonoid, String>(receiver = StringMonoid, block = local fun StringMonoid.<anonymous>(): String {
-    return listOf<String>(elements = ["O", "K"]).sum<String>(<this> = $this$with)
+    return listOf<String>(elements = ["O", "K"]).sum<String>(_context_receiver_0 = $this$with)
   }
 )
 }