[FIR] Highlight the weirdness of one test

The thing is, in a box test there is a proper
`UNRESOLVED_REFERENCE` on `M2().a.m2()` in K1 as well,
but it's not present in diagnostic tests. And also,
there's no error in the Kotlin IntelliJ plugin when
viewing such code.

It looks dangerous, but since this code is about
classes with identical fqNames, and since nothing
sus is really compiled, we can probably ignore this
problem. It's unlikely that it impacts much.

compiler/testData/diagnostics/tests/multimodule/duplicateClass/members.fir.kt

@@ -31,6 +31,7 @@ import p.*
 
 fun test(a: A) {
     a.m1()
+    a.<!UNRESOLVED_REFERENCE!>m2<!>()
 
     M1().a.m1()
 

compiler/testData/diagnostics/tests/multimodule/duplicateClass/members.kt

@@ -31,6 +31,7 @@ import p.*
 
 fun test(a: A) {
     a.m1()
+    a.<!UNRESOLVED_REFERENCE!>m2<!>()
 
     M1().a.m1()