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/passingLambdaToContextualParam.kt.txt

@@ -16,8 +16,8 @@ fun contextual(f: @ContextFunctionTypeParams(count = 1) Function1<C, String>): S
 }
 
 fun box(): String {
-  return contextual(f = local fun <anonymous>(<this>: C): String {
-    return <this>.<get-result>()
+  return contextual(f = local fun <anonymous>(_context_receiver_0: C): String {
+    return _context_receiver_0.<get-result>()
   }
 )
 }