Fix bug where attribute names unsuffixed themselves and shadowed things

Guava's multimaps return mutable collections from `get`, and
modifications to those collections are reflected in the multimap.

In this case, the scenario was:
- you're trying to find the declaration with attribute reference `Foo`
- you search for `Foo` and get an empty collection back
- you search for `FooAttribute` and add the results to that collection
- the scope now has a declaration called `Foo` mapping to `FooAttribute`

tl;dr mutability is evil.

Author: cirras

delphi-frontend/src/main/java/au/com/integradev/delphi/symbol/scope/DelphiScopeImpl.java

@@ -318,6 +318,7 @@ public Set<NameDeclaration> findDeclaration(NameOccurrence occurrence) {
 
     Set<NameDeclaration> found = declarationsByName.get(occurrence.getImage());
     if (occurrence.isAttributeReference()) {
+      found = new HashSet<>(found);
       found.addAll(declarationsByName.get(occurrence.getImage() + "Attribute"));
     }