Remove temporary allocations when calling cmGeneratorTarget::GetName.

This happens quite often from within comparisons such as in
NamedGeneratorTargetFinder or FindGeneratorTargetImpl. It is the
top hotspot of both, number of allocations as well as number of
temporary allocations - the majority of calls lead to temporary
allocations.

In raw numbers, this patch removes ~1E6 temporary allocations of
1.5E6 temporary allocations in total when running the cmake daemon
on the KDevelop build dir. That is 2/3 of the total.

This hotspot was found with heaptrack.

Source/cmGeneratorTarget.cxx

@@ -335,7 +335,7 @@ cmState::TargetType cmGeneratorTarget::GetType() const
 }
 
 //----------------------------------------------------------------------------
-std::string cmGeneratorTarget::GetName() const
+const std::string& cmGeneratorTarget::GetName() const
 {
   return this->Target->GetName();
 }

Source/cmGeneratorTarget.h

@@ -55,7 +55,7 @@ public:
     GetLinkInformation(const std::string& config) const;
 
   cmState::TargetType GetType() const;
-  std::string GetName() const;
+  const std::string& GetName() const;
   std::string GetExportName() const;
 
   std::vector<std::string> GetPropertyKeys() const;