LICENSE.TXT causes error while checking out repo on case-insensitive file system

[StoneSpb]

After merging 991cfd1 there are two license files in the repo: LICENSE.TXT and LICENSE.txt

When updating dependencies for swift development using utils/update-checkout script, first step seems to be cloning the llvm-project repo on default branch (next), then checking out desired branch/tag.
After cloning on case-insensitive file system (MacOS has the one by default) you can get a deleted file in your repo:

Cloning into 'llvm-project'...
Updating files:   3% (4732/121968)
...
Updating files: 100% (121968/121968), done.
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'LICENSE.TXT'
  'LICENSE.txt'

This causes the subsequent checkout operation to fail.

[benlangmuir]

Fixed by ecddb8c