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/Matcher.cpp

@@ -218,6 +218,17 @@ bool Matcher::match(const Template *type, const Template *otherType)
     return true;
 }
 
+bool Matcher::match(const ExplicitInstantiation *type, const ExplicitInstantiation *otherType)
+{
+    if (type == otherType)
+        return true;
+
+    if (! Matcher::match(type->name(), otherType->name(), this))
+        return false;
+
+    return true;
+}
+
 bool Matcher::match(const ForwardClassDeclaration *type, const ForwardClassDeclaration *otherType)
 {
     if (type == otherType)