FIR checker: report tailrec problems on the keyword

FE1.0 reports it on the declaration signature. This is not ideal so we
move it to the `tailrec` keyword in FIR.

compiler/testData/codegen/box/diagnostics/functions/tailRecursion/thisReferences.kt

@@ -19,7 +19,7 @@ class A {
         }
     }
 
-    <!NO_TAIL_CALLS_FOUND!>tailrec fun f3(a : A)<!> {
+    <!NO_TAIL_CALLS_FOUND!>tailrec<!> fun f3(a : A) {
         a.<!NON_TAIL_RECURSIVE_CALL!>f3<!>(a) // non-tail recursion, could be potentially resolved by condition if (a == this) f3() else a.f3()
     }
 }