Clear the link information in ClearLinkMaps.

The cache here needs to be cleared if GetLinkInformation is called
at configure-time, such as during an export().

The next commit does exactly that, and without this patch,
the LinkLanguage test would fail.

Source/cmTarget.cxx

@@ -1520,6 +1520,13 @@ void cmTarget::ClearLinkMaps()
   this->Internal->LinkImplMap.clear();
   this->Internal->LinkInterfaceMap.clear();
   this->Internal->LinkClosureMap.clear();
+  for (cmTargetLinkInformationMap::const_iterator it
+      = this->LinkInformation.begin();
+      it != this->LinkInformation.end(); ++it)
+    {
+    delete it->second;
+    }
+  this->LinkInformation.clear();
 }
 
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