liblangutil: SourceLocation: adds (shared) pointer to underlying CharStream source, eliminating sourceName

Also, adapted affecting code to those changes.

test/libevmasm/Optimiser.cpp

@@ -53,7 +53,7 @@ namespace
 		// add dummy locations to each item so that we can check that they are not deleted
 		AssemblyItems input = _input;
 		for (AssemblyItem& item: input)
-			item.setLocation(SourceLocation(1, 3, make_shared<string>("")));
+			item.setLocation(SourceLocation(1, 3, nullptr));
 		return input;
 	}