[CommandLine} Wire-up cl::list::setDefault() so it will work correctly with cl::Reset...

[CommandLine} Wire-up cl::list::setDefault() so it will work correctly with cl::ResetAllOptionOccurrences() in unittests.  Part 2 of 5

Summary:
With this change, cl::ResetAllOptionOccurrences() clears
cl::list just like cl::opt, allowing users to call
cl::ParseCommandLineOptions() multiple times without interference from
previous calls.

Reviewers: rnk

Reviewed By: rnk

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61234

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359522 91177308-0d34-0410-b5e6-96231b3b80d8