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/property.kt.txt

@@ -9,7 +9,6 @@ interface B {
 }
 
 val c: Int
-  get(<this>: A, <this>: B): Int {
-    return <this>.a().plus(other = <this>.b())
+  get(_context_receiver_0: A, _context_receiver_1: B): Int {
+    return _context_receiver_0.a().plus(other = _context_receiver_1.b())
   }
-