Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/inference/coercionToUnit/coercionToUnitForIfAsLastExpressionInLambda.fir.kt

@@ -12,7 +12,7 @@ fun foo(): String? {
     run {
         if (true) return@run
 
-        if (true) return Obj() // correct error, type check against return type of function "foo"
+        if (true) return <!RETURN_TYPE_MISMATCH!>Obj()<!> // correct error, type check against return type of function "foo"
     }
 
     run {

compiler/testData/diagnostics/tests/inference/coercionToUnit/noCoercion.fir.kt

@@ -23,4 +23,4 @@ fun noCoercionBlockHasExplicitReturn() {
     }
 }
 
-fun noCoercionInExpressionBody(): Unit = "hello"
+fun noCoercionInExpressionBody(): Unit = <!RETURN_TYPE_MISMATCH!>"hello"<!>