Changes

Fix use of field index in daily report
Links to graph plots in the daily report make use of the field index but 
haven't been updated to get it via get_field_index, causing these links
to be broken. Fix this by using get_field_index.
Differential Revision: https://reviews.llvm.org/D47581
 git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@333896
91177308-0d34-0410-b5e6-96231b3b80d8