C++: Ignore explicit template instantiations

Defined in section 14.7.2 of the standard.

Fixes completion for std::string.

The following explicit instantiation appears in bits/basic_string.tcc:
  extern template class basic_string<char>;

This is wrongfully considered a specialization for a forward declaration
(like `template<> class basic_string<char>` is).

Introduce a new Symbol type for explicit instantiations.

Use-case:
template<class T>
struct Foo { T bar; };

template class Foo<int>;

void func()
{
    Foo<int> foo;
    foo.bar; // bar not highlighted
}

Change-Id: I9e35c8c32f6b78fc87b4f4f1fc903b42cfbd2c2b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/3rdparty/cplusplus/Symbol.cpp

@@ -361,6 +361,9 @@ bool Symbol::isNamespace() const
 bool Symbol::isTemplate() const
 { return asTemplate() != 0; }
 
+bool Symbol::isExplicitInstantiation() const
+{ return asExplicitInstantiation() != 0; }
+
 bool Symbol::isClass() const
 { return asClass() != 0; }