Add CoreOption flag to "-coverage" option to make it available for clang-cl

Summary:
The -coverage option is not a CoreOption, so it is not available to clang-cl.
This patch adds the CoreOption flag to "-coverage" to allow it to be used with clang-cl.

Reviewers: rnk

Reviewed By: rnk

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38221

llvm-svn: 317709