ClangCodeModel: Find usage lists usages for included Qt library headers

Steps to reproduce:

• start Creator with enabled ClangCodeModel, set СlangCodelModel as default for C++
• create a simple Qt Application from template
• place cursor onto a Qt class
• trigger Find usages... (Ctrl+Shift+U)

Expected behavior:

• result lists usages only from current project(s) as it's done by Qt Creator's default builtin code model

Current behavior:

• result lists usages of the class even inside the Qt headers