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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@281873 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 6360ab9da47db7cd5c4841b39568c8a1ae11e836)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm