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/Symbols.h

@@ -423,8 +423,41 @@ protected:
     virtual void visitSymbol0(SymbolVisitor *visitor);
     virtual void accept0(TypeVisitor *visitor);
     virtual bool match0(const Type *otherType, Matcher *matcher) const;
+
+private:
+    bool _isExplicitInstantiation;
 };
 
+class CPLUSPLUS_EXPORT ExplicitInstantiation : public Scope, public Type
+{
+public:
+    ExplicitInstantiation(TranslationUnit *translationUnit, unsigned sourceLocation, const Name *name);
+    ExplicitInstantiation(Clone *clone, Subst *subst, ExplicitInstantiation *original);
+    virtual ~ExplicitInstantiation();
+
+    Symbol *declaration() const;
+
+    // Symbol's interface
+    virtual FullySpecifiedType type() const;
+
+    virtual const ExplicitInstantiation *asExplicitInstantiation() const
+    { return this; }
+
+    virtual ExplicitInstantiation *asExplicitInstantiation()
+    { return this; }
+
+    // Type's interface
+    virtual const ExplicitInstantiation *asExplicitInstantiationType() const
+    { return this; }
+
+    virtual ExplicitInstantiation *asExplicitInstantiationType()
+    { return this; }
+
+protected:
+    virtual void visitSymbol0(SymbolVisitor *visitor);
+    virtual void accept0(TypeVisitor *visitor);
+    virtual bool match0(const Type *otherType, Matcher *matcher) const;
+};
 
 class CPLUSPLUS_EXPORT Namespace: public Scope, public Type
 {