[LL] find fir by psi: allow fallthrough to traverse tree for invalid code

In case of broken code e.g., duplicated classes provider would return first class,
though we definitely need some code insight in this case
at least to add navigation fixes, etc.
Similar to this, a file copy is created during completion,
where additional elements might appear and we need to search for them.

Added test cases for duplicated classes.

analysis/low-level-api-fir/testdata/getOrBuildFir/invalidCode/duplicatedClasses.kt

@@ -0,0 +1,6 @@
+class SomeClass {
+    val prop = "abc"
+}
+class SomeClass {
+    <expr>fun foo() {}</expr>
+}