[ATR-22206] Change InputMaker merging code to be typeless

This MR changes matchDecision from a templated function on the link CONTAINER type, to a typeless function which manipulates type data via the type's CLID.

This resolves a limitation where a user needing to use m_mergeUsingFeature could only do so if their slice's feature inherited from xAOD::IParticle. This limitation is now gone.

Note: I am getting a linker error against the typelessfindLinks function which I don't understand. Running CI to check if it's just me.

Trigger/TrigSteer/DecisionHandling/DecisionHandling/InputMakerBase.h

@@ -47,8 +47,7 @@ This is a base class for HLT InputMakers to reduce boilerplate and enforce the c
   /// does the standard handling of input decisions: read from handles with all the checks, create merged output handles and link them, copies links and return outputHandles
   StatusCode decisionInputToOutput(const EventContext& context, SG::WriteHandle<TrigCompositeUtils::DecisionContainer>& outputHandle) const;
 
-  /// Checks for merge-able Decision objects coming from N upstream filters. Check based on stored element link in typed CONTAINER with 'linkNameToMatch'
-  template<typename CONTAINER>
+  /// Checks for merge-able Decision objects coming from N upstream filters. Check based on most-recent element link with name 'linkNameToMatch'. Works for any link type.
   size_t matchDecision(const TrigCompositeUtils::DecisionContainer* outDecisions, 
     const TrigCompositeUtils::Decision* toMatch, 
     const std::string& linkNameToMatch) const;
@@ -68,6 +67,4 @@ This is a base class for HLT InputMakers to reduce boilerplate and enforce the c
   
 };
 
-#include "InputMakerBase.icc"
-
 #endif // DECISIONHANDLING_INPUTMAKERBASE_H