cmTarget: Refactor internal LinkImplementation map

If ComputeLinkImplementationLanguages were ever to cause
GetLinkImplementationLibraries to be invoked then a LinkImplMap entry
may appear in the middle of computing it in GetLinkInformation.  Instead
create the map entry up front and store in it boolean values indicating
which pieces of the LinkImplementation structure have been populated.
This approach leads to shorter code that is easier to follow too.

Source/cmTarget.cxx

@@ -144,8 +144,15 @@ public:
   CompileInfoMapType CompileInfoMap;
 
   // Cache link implementation computation from each configuration.
+  struct OptionalLinkImplementation: public cmTarget::LinkImplementation
+  {
+    OptionalLinkImplementation():
+      LibrariesDone(false), LanguagesDone(false) {}
+    bool LibrariesDone;
+    bool LanguagesDone;
+  };
   typedef std::map<TargetConfigPair,
-                   cmTarget::LinkImplementation> LinkImplMapType;
+                   OptionalLinkImplementation> LinkImplMapType;
   LinkImplMapType LinkImplMap;
 
   typedef std::map<std::string, cmTarget::LinkClosure> LinkClosureMapType;
@@ -6519,28 +6526,21 @@ cmTarget::GetLinkImplementation(const std::string& config) const
     return 0;
     }
 
-  // Lookup any existing link implementation for this configuration.
+  // Populate the link implementation for this configuration.
   TargetConfigPair key(this, cmSystemTools::UpperCase(config));
-
-  cmTargetInternals::LinkImplMapType::iterator
-    i = this->Internal->LinkImplMap.find(key);
-  if(i == this->Internal->LinkImplMap.end())
+  cmTargetInternals::OptionalLinkImplementation&
+    impl = this->Internal->LinkImplMap[key];
+  if(!impl.LibrariesDone)
     {
-    // Compute the link implementation for this configuration.
-    LinkImplementation impl;
+    impl.LibrariesDone = true;
     this->ComputeLinkImplementation(config, impl, this);
-    this->ComputeLinkImplementationLanguages(config, impl, this);
-
-    // Store the information for this configuration.
-    cmTargetInternals::LinkImplMapType::value_type entry(key, impl);
-    i = this->Internal->LinkImplMap.insert(entry).first;
     }
-  else if (i->second.Languages.empty())
+  if(!impl.LanguagesDone)
     {
-    this->ComputeLinkImplementationLanguages(config, i->second, this);
+    impl.LanguagesDone = true;
+    this->ComputeLinkImplementationLanguages(config, impl, this);
     }
-
-  return &i->second;
+  return &impl;
 }
 
 //----------------------------------------------------------------------------
@@ -6561,23 +6561,16 @@ cmTarget::GetLinkImplementationLibrariesInternal(const std::string& config,
     return 0;
     }
 
-  // Lookup any existing link implementation for this configuration.
+  // Populate the link implementation libraries for this configuration.
   TargetConfigPair key(head, cmSystemTools::UpperCase(config));
-
-  cmTargetInternals::LinkImplMapType::iterator
-    i = this->Internal->LinkImplMap.find(key);
-  if(i == this->Internal->LinkImplMap.end())
+  cmTargetInternals::OptionalLinkImplementation&
+    impl = this->Internal->LinkImplMap[key];
+  if(!impl.LibrariesDone)
     {
-    // Compute the link implementation for this configuration.
-    LinkImplementation impl;
-    this->ComputeLinkImplementation(config, impl, head);
-
-    // Store the information for this configuration.
-    cmTargetInternals::LinkImplMapType::value_type entry(key, impl);
-    i = this->Internal->LinkImplMap.insert(entry).first;
+    impl.LibrariesDone = true;
+    this->ComputeLinkImplementation(config, impl, this);
     }
-
-  return &i->second;
+  return &impl;
 }
 
 //----------------------------------------------------------------------------