Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/smartCasts/elvisExprNotNull.fir.kt

@@ -33,5 +33,5 @@ fun baz(s: String?, r: String?): String {
 fun withNull(s: String?): String {
     val t = s ?: null
     // Error: nullable
-    return t
+    return <!RETURN_TYPE_MISMATCH!>t<!>
 }

compiler/testData/diagnostics/tests/smartCasts/propertyToNotNull.fir.kt

@@ -8,7 +8,7 @@ class Immutable(val x: String?) {
 
 class Mutable(var y: String?) {
     fun foo(): String {
-        if (y != null) return y
+        if (y != null) return <!RETURN_TYPE_MISMATCH!>y<!>
         return ""
     }
 }

compiler/testData/diagnostics/tests/smartCasts/safecalls/falseSecondArgument.fir.kt

@@ -3,5 +3,5 @@
 fun calc(x: List<String>?, y: Int?): Int {
     x?.subList(y!! - 1, y)
     // y!! above should not provide smart cast here
-    return y
+    return <!RETURN_TYPE_MISMATCH!>y<!>
 }

compiler/testData/diagnostics/tests/smartCasts/safecalls/receiverAndChainFalse.fir.kt

@@ -6,5 +6,5 @@ fun calc(x: String?, y: Int?): Int {
     // Smart cast because of x!! in receiver
     foo(x!!)?.subSequence(y!!, x.length)?.length
     // No smart cast possible
-    return y
+    return <!RETURN_TYPE_MISMATCH!>y<!>
 }

compiler/testData/diagnostics/tests/smartCasts/varnotnull/forEach.fir.kt

@@ -13,5 +13,5 @@ fun list(start: SomeObject): SomeObject {
         e = e<!UNSAFE_CALL!>.<!>next()
     }
     // Smart cast is not possible here due to next()
-    return e
+    return <!RETURN_TYPE_MISMATCH!>e<!>
 }

compiler/testData/diagnostics/tests/smartCasts/varnotnull/forEachWithBreak.fir.kt

@@ -14,5 +14,5 @@ fun list(start: SomeObject): SomeObject {
         e.doSomething()
         e = e.next()
     }
-    return e
+    return <!RETURN_TYPE_MISMATCH!>e<!>
 }

compiler/testData/diagnostics/tests/smartCasts/varnotnull/forEachWithContinue.fir.kt

@@ -14,5 +14,5 @@ fun list(start: SomeObject): SomeObject {
         e.doSomething()
         e = e.next()
     }
-    return e
+    return <!RETURN_TYPE_MISMATCH!>e<!>
 }