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/overloading.kt.txt

@@ -1,14 +1,14 @@
-fun foo(<this>: Int, <this>: String): Int {
-  return <this>.plus(other = 42)
+fun foo(_context_receiver_0: Int, _context_receiver_1: String): Int {
+  return _context_receiver_0.plus(other = 42)
 }
 
-fun foo(<this>: Int): Int {
-  return <this>.plus(other = 42)
+fun foo(_context_receiver_0: Int): Int {
+  return _context_receiver_0.plus(other = 42)
 }
 
 fun test() {
   with<Int, Int>(receiver = 42, block = local fun Int.<anonymous>(): Int {
-    return foo(<this> = $this$with)
+    return foo(_context_receiver_0 = $this$with)
   }
 ) /*~> Unit */
 }