VS: Add method to re-parse specific option parser fields

This will allow a client to parse flags, replace the flag tables, and
then re-parse a field in which flags for a secondary tool were
collected.

Source/cmVisualStudioGeneratorOptions.h

@@ -52,6 +52,10 @@ public:
 
   void PrependInheritedString(std::string const& key);
 
+  // Parse the content of the given flag table entry again to extract
+  // known flags and leave the rest in the original entry.
+  void Reparse(std::string const& key);
+
   // Fix the ExceptionHandling option to default to off.
   void FixExceptionHandlingDefault();
 
@@ -84,6 +88,8 @@ private:
   bool FortranRuntimeDLL;
   bool FortranRuntimeMT;
 
+  std::string UnknownFlagField;
+
   virtual void StoreUnknownFlag(const char* flag);
 };