Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/deadCodeDifferentExamples.fir.kt

@@ -6,7 +6,7 @@ fun t1() : Int{
 }
 
 fun t1a() : Int {
-  return
+  <!RETURN_TYPE_MISMATCH!>return<!>
   return 1
   1
 }
@@ -19,7 +19,7 @@ fun t1b() : Int {
 
 fun t1c() : Int {
   return 1
-  return
+  <!RETURN_TYPE_MISMATCH!>return<!>
   1
 }
 

compiler/testData/diagnostics/tests/controlFlowAnalysis/elvisNotProcessed.fir.kt

@@ -35,5 +35,5 @@ val bbbb = ( l() ?: null) ?: ( l() ?: null)
 
 fun f(x : Long?): Long {
     var a = x ?: (fun() {} ?: fun() {})
-    return a
+    return <!RETURN_TYPE_MISMATCH!>a<!>
 }