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/labeledThisReferences.kt

@@ -16,7 +16,7 @@ class B {
             }
         }
 
-        <!NO_TAIL_CALLS_FOUND!>tailrec fun h2(x : Any)<!> {
+        <!NO_TAIL_CALLS_FOUND!>tailrec<!> fun h2(x : Any) {
             this@B.h2("no recursion") // keep vigilance
         }