[FIR] Implement FirReturnAllowedChecker

Supported diagnostics:
- RETURN_NOT_ALLOWED
- RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY

compiler/testData/diagnostics/tests/controlStructures/kt799.fir.kt

@@ -12,11 +12,11 @@ fun test() {
     doSmth(if (true) 3 else return, <!TOO_MANY_ARGUMENTS!>1<!>)
 }
 
-val a : Nothing = return 1
+val a : Nothing = <!RETURN_NOT_ALLOWED!>return<!> 1
 
-val b = return 1
+val b = <!RETURN_NOT_ALLOWED!>return<!> 1
 
-val c = doSmth(if (true) 3 else return)
+val c = doSmth(if (true) 3 else <!RETURN_NOT_ALLOWED!>return<!>)
 
 
 fun f(mi: Int = if (true) 0 else return) {}