[FIR] Implement FirReturnAllowedChecker

Supported diagnostics:
- RETURN_NOT_ALLOWED
- RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY

compiler/testData/diagnostics/tests/labels/kt361.fir.kt

@@ -1,11 +0,0 @@
-fun nonlocals(b : Boolean) {
-    a@{
-        fun foo() {
-            if (b) {
-                return@a 1 // The label must be resolved, but an error should be reported for a non-local return
-            }
-        }
-
-        return@a 5
-    }
-}

compiler/testData/diagnostics/tests/labels/kt361.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun nonlocals(b : Boolean) {
     a@{
         fun foo() {