[FIR] Fix inconsistent RETURN_TYPE_MISMATCH and TYPE_MISMATCH reporting on functions and properties

^KT-51203 Fixed

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

@@ -9,4 +9,4 @@ fun case1(a: Boolean) = run { println("d"); return <!RETURN_TYPE_MISMATCH!>true<
 
 // TESTCASE NUMBER: 2
 val case2
-get() = run { println("d"); return true }
+get() = run { println("d"); return <!RETURN_TYPE_MISMATCH!>true<!> }

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

@@ -30,7 +30,7 @@ class Case6 {
 
 // TESTCASE NUMBER: 7
 class Case7 {
-    val x: Int get() = null
+    val x: Int get() = <!NULL_FOR_NONNULL_TYPE!>null<!>
 }
 
 // TESTCASE NUMBER: 8