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.h

@@ -135,7 +135,7 @@ public:
     /// Returns true if this Symbol is an Enum.
     bool isEnum() const;
 
-    /// Returns true if this Symbol is an Function.
+    /// Returns true if this Symbol is a Function.
     bool isFunction() const;
 
     /// Returns true if this Symbol is a Namespace.
@@ -144,6 +144,9 @@ public:
     /// Returns true if this Symbol is a Template.
     bool isTemplate() const;
 
+    /// Returns true if this Symbol is an ExplicitInstantiation.
+    bool isExplicitInstantiation() const;
+
     /// Returns true if this Symbol is a Class.
     bool isClass() const;
 
@@ -203,6 +206,7 @@ public:
     virtual const Function *asFunction() const { return 0; }
     virtual const Namespace *asNamespace() const { return 0; }
     virtual const Template *asTemplate() const { return 0; }
+    virtual const ExplicitInstantiation *asExplicitInstantiation() const { return 0; }
     virtual const NamespaceAlias *asNamespaceAlias() const { return 0; }
     virtual const Class *asClass() const { return 0; }
     virtual const Block *asBlock() const { return 0; }
@@ -229,6 +233,7 @@ public:
     virtual Function *asFunction() { return 0; }
     virtual Namespace *asNamespace() { return 0; }
     virtual Template *asTemplate() { return 0; }
+    virtual ExplicitInstantiation *asExplicitInstantiation() { return 0; }
     virtual NamespaceAlias *asNamespaceAlias() { return 0; }
     virtual Class *asClass() { return 0; }
     virtual Block *asBlock() { return 0; }