[FIR] Implement ASSIGNMENT_TYPE_MISMATCH, RESULT_TYPE_MISMATCH diagnostics, fix tests

compiler/testData/diagnostics/tests/generics/varProjection/setterProjectedOutNoPlusAssign.fir.kt

@@ -7,5 +7,5 @@ interface Tr<T> {
 fun test(t: Tr<out String>) {
     // resolved as t.v = t.v + null!!, where type of right operand is String,
     // so TYPE_MISMATCH: String is not <: of Captured(out String)
-    t.v += null!!
-}
+    <!ASSIGNMENT_TYPE_MISMATCH!>t.v += null!!<!>
+}