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/diagnostics/tests/tailRecSingleton.fir.kt

@@ -44,7 +44,7 @@ enum class E {
         }
     },
     C {
-        <!NO_TAIL_CALLS_FOUND!>override tailrec fun rec()<!> {
+        override <!NO_TAIL_CALLS_FOUND!>tailrec<!> fun rec() {
             C.rec() // resolution goes to `E.rec`. Hence the resolved symbol is considered different from `C.rec`.
         }
     };