Report locations for RGBLINK errors with conflicting objects (#1494)

This requires updating the object file format to record the
fstack context for sections themselves, not just for patches.

include/link/symbol.hpp

@@ -25,7 +25,6 @@ struct Symbol {
 	// Info contained in the object files
 	std::string name;
 	ExportLevel type;
-	char const *objFileName;
 	FileStackNode const *src;
 	int32_t lineNo;
 	Either<