Allow -fsanitize-coverage=N with ubsan, clang part

Summary:
Allow -fsanitize-coverage=N with ubsan, clang part.
This simply allows the flag combination.
The LLVM will work out of the box, the compile-rt part
will follow as a separate patch.

Test Plan: check-clang

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6849

llvm-svn: 225229