GeneratorObjects: Fix compilation with c++20.

With c++20, GenParticle*==HepMcParticleLink was ambiguous.

 - The second argument could convert to GenParticle* and then compare using
   the built-in == operation.
 - The first argument could convert to HepMcParticleLink and the compare
   using HepMcParticleLink::operator==.

The second of these is only allowed in c++20 due to the fact that operator==
is implicitly reversable (otherwise, we wouldn't be able to convert the
first argument as this is a member function).

Add an explicit operator== for this case to disambiguate (choosing the first
alternative).

Generators/GeneratorObjects/GeneratorObjects/HepMcParticleLink.h

 /*
-  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef GENERATOROBJECTS_HEPMCPARTICLELINK_H
@@ -665,6 +665,16 @@ public:
 };
 
 
+/**
+ * @brief Comparison with GenParticle*.
+ * Needed with c++20 to break an ambiguity between the built-in GenParticle*
+ * equality and operator== defined above, arising due to the conversions
+ * back and forth between HepMcParticleLink and GenParticle*.
+ */
+bool operator== (const HepMC::GenParticle* a,
+                 const HepMcParticleLink& b);
+
+
 /**
  * @brief Output operator.
  * @param os Stream to which to output.

Generators/GeneratorObjects/GeneratorObjects/HepMcParticleLink.icc

 /*
- * Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration.
+ * Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration.
  */
 /**
  * @file GeneratorObjects/HepMcParticleLink.icc
@@ -600,3 +600,17 @@ HepMcParticleLink::barcode_type HepMcParticleLink::compress() const
   return ( ((m_extBarcode.barcode()&0xFFFF) << 16) |
            eventIndex() );
 }
+
+
+/**
+ * @brief Comparison with GenParticle*.
+ * Needed with c++20 to break an ambiguity between the built-in GenParticle*
+ * equality and operator== defined above, arising due to the conversions
+ * back and forth between HepMcParticleLink and GenParticle*.
+ */
+inline
+bool operator== (const HepMC::GenParticle* a,
+                 const HepMcParticleLink& b)
+{
+  return a == b.cptr();
+}