Replace some FIR syntax errors with more proper diagnostics

compiler/fir/analysis-tests/testData/resolve/diagnostics/superIsNotAnExpression.kt

@@ -7,7 +7,7 @@ class B: A() {
         <!UNRESOLVED_REFERENCE!>invoke<!>()
 
         <!SUPER_IS_NOT_AN_EXPRESSION!>super<!> {
-            println('weird')
+            println(<!ILLEGAL_CONST_EXPRESSION!>'weird'<!>)
         }
     }
 }

compiler/fir/analysis-tests/testData/resolve/expresssions/operators/plusAndPlusAssign.kt

@@ -5,5 +5,5 @@ class Foo {
 
 fun test() {
     var f = Foo()
-    <!ASSIGN_OPERATOR_AMBIGUITY, ASSIGN_OPERATOR_AMBIGUITY, ASSIGN_OPERATOR_AMBIGUITY!>f += f<!>
+    <!ASSIGN_OPERATOR_AMBIGUITY!>f += f<!>
 }