C++: Fix decltype resolving for template function

The last nail for std::unique_ptr (GCC variant, MSVC still doesn't work).

Use-case:
template<typename T>
static T f();

struct Foo { int bar; };

void fun()
{
    decltype(f<Foo>()) s;
    s.bar; // bar not highlighted
}

Task-number: QTCREATORBUG-14483
Task-number: QTCREATORBUG-8937
Change-Id: I5bab757400b070cf9dbb688a44fd8eafe95ddc61
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/LookupContext.cpp

@@ -967,7 +967,7 @@ void CreateBindings::lookupInScope(const Name *name, Scope *scope,
             if (const TemplateNameId *instantiation = name->asTemplateNameId()) {
                 if (Template *specialization = s->asTemplate()) {
                     if (const Symbol *decl = specialization->declaration()) {
-                        if (decl->isFunction()) {
+                        if (decl->isFunction() || decl->isDeclaration()) {
                             Clone cloner(_control.data());
                             Subst subst(_control.data());
                             initializeSubst(cloner, subst, binding, scope,