[Driver][test] Avoid undefined grep in darwin-ld.c

Summary:
question-mark is not a BRE special character.

POSIX.1-2017 XBD Section 9.3.2 indicates that the interpretation of `\?`
as used by rC366282 is undefined. This patch uses an ERE instead.

Reviewers: rnk, daltenty, xingxue, jasonliu

Reviewed By: rnk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65668

llvm-svn: 367709