cmInstallExportGenerator: add a method for the file destination

This will be used in the error message which remarks about a target
being exported ambiguously into different installations.

Source/cmInstallExportGenerator.cxx

@@ -218,3 +218,8 @@ void cmInstallExportGenerator::GenerateScriptActions(std::ostream& os,
                        false, this->FilePermissions.c_str(), nullptr, nullptr,
                        nullptr, indent);
 }
+
+std::string cmInstallExportGenerator::GetDestinationFile() const
+{
+  return this->Destination + '/' + this->FileName;
+}

Source/cmInstallExportGenerator.h

@@ -41,6 +41,7 @@ public:
   const std::string& GetNamespace() const { return this->Namespace; }
 
   std::string const& GetDestination() const { return this->Destination; }
+  std::string GetDestinationFile() const;
 
 protected:
   void GenerateScript(std::ostream& os) override;