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/function.kt.txt

@@ -11,13 +11,13 @@ class C {
 
 }
 
-fun foo(<this>: C) {
-  <this>.<get-c>() /*~> Unit */
+fun foo(_context_receiver_0: C) {
+  _context_receiver_0.<get-c>() /*~> Unit */
 }
 
 fun bar(c: C) {
   with<C, Unit>(receiver = c, block = local fun C.<anonymous>() {
-    foo(<this> = $this$with)
+    foo(_context_receiver_0 = $this$with)
   }
 )
 }