ignore .orig and .rej files (iree-org#16359)

These are typically created by the `patch` command.

For exampe in iree-org#16358 i'm removing an
accidentally checked-in .orig file.

.gitignore

@@ -90,3 +90,7 @@ llvm-external-projects/iree-dialects/.cache
 
 # pkgci artifacts
 artifacts/
+
+# Side effects of local patching/merging
+*.orig
+*.rej