[llvm-cov] Make 'adjustColumnWidths' do less work

This drops some redundant calls to get{UniqueSourceFiles,
CoveredFunctions}. We can figure out the right column widths without
re-doing this expensive work.

This isn't NFC, but I don't want to check in another binary *.covmapping
file with long filenames in it. I tested this locally on a project with
some long filenames (FileCheck).

llvm-svn: 281873