[llvm] Ignore .rej files in .gitignore

Ignore reject files (.rej) files generated by patch. I can't imagine a
reason they should ever be checked in. I considered ignoring patch files
as well but decided to err on the side of caution because we might not
want them to be easily deleted by something like git clean.

Differential revision: https://reviews.llvm.org/D124619

.gitignore

@@ -13,6 +13,8 @@
 *~
 # Merge files created by git.
 *.orig
+# Reject files created by patch.
+*.rej
 # Byte compiled python modules.
 *.pyc
 # vim swap files