lnttool: Fix time computation in 'lnt send-daily-report' --today option.

 - The time we use to compute the report time should be in UTC, since that is
   what the daily report does its computation in.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@179706 91177308-0d34-0410-b5e6-96231b3b80d8