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/tailRecursionComplex.kt

@@ -5,7 +5,7 @@ object O {
 }
 
 class A {
-    <!NO_TAIL_CALLS_FOUND!>tailrec fun foo(i: Int)<!> = if (i < 0) 0 else A.foo(i - 1)
+    <!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo(i: Int) = if (i < 0) 0 else A.foo(i - 1)
 
     companion object {
         fun foo(i: Int) = 42 + i
@@ -13,5 +13,5 @@ class A {
 }
 
 class B {
-    <!NO_TAIL_CALLS_FOUND!>tailrec fun foo(i: Int)<!> = if (i < 0) 0 else O.foo(i - 1)
-}
+    <!NO_TAIL_CALLS_FOUND!>tailrec<!> fun foo(i: Int) = if (i < 0) 0 else O.foo(i - 1)
+}