[FIR] KT-58460: Report a return type mismatch diagnostic

^KT-58460 Fixed

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirFunctionReturnTypeMismatchChecker.kt

@@ -86,6 +86,16 @@ object FirFunctionReturnTypeMismatchChecker : FirReturnExpressionChecker() {
                     )
                 }
             }
+        } else if (resultExpression.source?.kind == KtFakeSourceElementKind.ImplicitUnit && !functionReturnType.isUnit) {
+            reporter.reportOn(
+                resultExpression.source,
+                RETURN_TYPE_MISMATCH,
+                functionReturnType,
+                returnExpressionType,
+                targetElement,
+                false,
+                context
+            )
         }
     }
 }

compiler/testData/diagnostics/tests/FunctionReturnTypes.fir.kt

@@ -7,9 +7,9 @@ fun unitEmpty() : Unit {}
 fun unitEmptyReturn() : Unit {return}
 fun unitIntReturn() : Unit {return <!RETURN_TYPE_MISMATCH!>1<!>}
 fun unitUnitReturn() : Unit {return Unit}
-fun test1() : Any = {<!RETURN_NOT_ALLOWED!>return<!>}
+fun test1() : Any = {<!RETURN_NOT_ALLOWED, RETURN_TYPE_MISMATCH!>return<!>}
 fun test2() : Any = a@ {return@a 1}
-fun test3() : Any { return }
+fun test3() : Any { <!RETURN_TYPE_MISMATCH!>return<!> }
 fun test4(): ()-> Unit = { <!RETURN_NOT_ALLOWED, RETURN_TYPE_MISMATCH!>return@test4<!> }
 fun test5(): Any = l@{ return@l }
 fun test6(): Any = {<!RETURN_NOT_ALLOWED!>return<!> 1}

compiler/testData/diagnostics/tests/funReturnsAny.fir.kt

@@ -1,5 +0,0 @@
-// ISSUE: KT-58460
-
-fun someFunction() : Any {
-    return
-}

compiler/testData/diagnostics/tests/funReturnsAny.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // ISSUE: KT-58460
 
 fun someFunction() : Any {