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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317709 91177308-0d34-0410-b5e6-96231b3b80d8