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/tailRecWithExtensionReceiver.kt

@@ -7,7 +7,7 @@ tailrec fun String.foo2() {
     this.foo2()
 }
 
-<!NO_TAIL_CALLS_FOUND!>tailrec fun String.foo3()<!> {
+<!NO_TAIL_CALLS_FOUND!>tailrec<!> fun String.foo3() {
     with(this) {
         <!NON_TAIL_RECURSIVE_CALL!>foo3<!>()
     }