C++: Fix "reference" file in symbol finder

Througout the initial review the singleton was transformed into an
ordinary class, but a error was introduced: The "reference" file was
incorrectly assumed to be the editors file, which is wrong, since
it should be the declaration file.

Change-Id: Iad3e25a690fa8bd07a18184b24b10f8dea965332
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/plugins/designer/qtcreatorintegration.cpp

@@ -274,8 +274,7 @@ static Document::Ptr findDefinition(Function *functionDeclaration, int *line)
 {
     if (CppModelManagerInterface *cppModelManager = CppModelManagerInterface::instance()) {
         const Snapshot snapshot = cppModelManager->snapshot();
-        CppTools::SymbolFinder symbolFinder(functionDeclaration->fileName(),
-                                            functionDeclaration->fileNameLength());
+        CppTools::SymbolFinder symbolFinder;
         if (Symbol *def = symbolFinder.findMatchingDefinition(functionDeclaration, snapshot)) {
             if (line)
                 *line = def->line();