C++: fix matching type with using from other namespace

example code:
struct S { int s; };

namespace std
{
    template <typename T>
    struct shared_ptr
    {
        T* operator->();
    };
}

namespace NS
{
    using std::shared_ptr;
}

int main()
{
    NS::shared_ptr<S> p;// for this shared_ptr
    return 0;
}

Fixes:
* find usages
* follow symbol
* highlighting
* marking

Task-number: QTCREATORBUG-7978
Change-Id: I28994c960b87ddd400e1d7b860fca6c6683bbb5a
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppTools: Tests: Introduce basic BuiltinSymbolSearcher tests

Change-Id: I25f11e80747863f797d146980d8ee0e605acb6e5
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Tests: Extract TestDataDir to the coreplugin

Change-Id: Ie290c07c07c13134a57f328e9ae876b2af6974db
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>