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/section.hpp

@@ -46,6 +46,8 @@ struct Section {
 	bool isAlignFixed;
 	uint16_t alignMask;
 	uint16_t alignOfs;
+	FileStackNode const *src;
+	int32_t lineNo;
 	std::vector<uint8_t> data; // Array of size `size`, or 0 if `type` does not have data
 	std::vector<Patch> patches;
 	// Extra info computed during linking

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<