Quote '?' in llvm-rc test

Summary:
Bash interperets the '?' character as matching an arbitrary character.
On systems that have a file or directory with exactly one character in
their root directory, '/?' gets reinterpreted into that pathname, which
fails to match the expected Help text for llvm-rc.
This patch quotes the '/?' to avoid that edge case.

Reviewers: mnbvmar, ecbeckmann, rnk

Reviewed By: rnk

Subscribers: dyung, ruiu, llvm-commits

Differential Revision: https://reviews.llvm.org/D35852

llvm-svn: 309133