Introduce FirFunctionReturnTypeMismatchChecker

compiler/tests-spec/testData/diagnostics/linked/expressions/jump-expressions/return-expressions/p-4/neg/1.1.fir.kt

@@ -5,7 +5,7 @@
 // TESTCASE NUMBER: 1
 // UNEXPECTED BEHAVIOUR
 // ISSUES : KT-35545
-fun case1(a: Boolean) = run { println("d"); return true }
+fun case1(a: Boolean) = run { println("d"); return <!RETURN_TYPE_MISMATCH!>true<!> }
 
 // TESTCASE NUMBER: 2
 val case2

compiler/tests-spec/testData/diagnostics/linked/type-system/introduction-1/p-6/neg/2.2.fir.kt

@@ -43,13 +43,13 @@ class Case8 {
 }
 
 // TESTCASE NUMBER: 9
-fun case_9(): Any = null
+fun case_9(): Any = <!RETURN_TYPE_MISMATCH!>null<!>
 
 // TESTCASE NUMBER: 10
-fun case_10(x: Int, y: Boolean): Any = if (y) x else null
+fun case_10(x: Int, y: Boolean): Any = <!RETURN_TYPE_MISMATCH!>if (y) x else null<!>
 
 // TESTCASE NUMBER: 11
-fun case_11(x: Int, y: Boolean): Any = if (y) x else null
+fun case_11(x: Int, y: Boolean): Any = <!RETURN_TYPE_MISMATCH!>if (y) x else null<!>
 
 // TESTCASE NUMBER: 12
 class Case12 {

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/45.fir.kt

@@ -17,5 +17,5 @@ fun case_1(x: Number?): Long? {
  * ISSUES: KT-22997
  */
 fun case_2(x: Number?): Long? {
-    if (x == null || x is Long) return x else return 0L
+    if (x == null || x is Long) return <!RETURN_TYPE_MISMATCH!>x<!> else return 0L
 }

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/64.fir.kt

@@ -17,7 +17,7 @@ class Case1<T> {
             x = getT()
         }
         <!DEBUG_INFO_EXPRESSION_TYPE("T?")!>x<!>
-        return x
+        return <!RETURN_TYPE_MISMATCH!>x<!>
     }
 }