[llvm-cov] Remove "default:" label in the switch covering all enum values.

Summary:
Fixing the build breakage:
http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/27309

Reviewers: vsk, allevato, Dor1s

Reviewed By: Dor1s

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54323

llvm-svn: 346514