Fix libc++ configuration with -fsanitize-coverage

Summary:
a recent change (r280015) in libc++ configuration broke LibFuzzer bot:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer/builds/12245

It's not restricted just to that bot; any code that uses the sanitize coverage and configures libc++ hits it.

This CL fixes the issue.

Reviewers: compnerd

Subscribers: aizatsky

Differential Revision: https://reviews.llvm.org/D24116

llvm-svn: 280335