[lit] Remove TODO

Summary:
Instead of keeping track of TODOs for lit in a file checked into source
control, use LLVM's bug tracker. The TODOs have been migrated to the
following bugs:

* https://llvm.org/bugs/show_bug.cgi?id=30666
* https://llvm.org/bugs/show_bug.cgi?id=30667
* https://llvm.org/bugs/show_bug.cgi?id=30668
* https://llvm.org/bugs/show_bug.cgi?id=30669
* https://llvm.org/bugs/show_bug.cgi?id=30670
* https://llvm.org/bugs/show_bug.cgi?id=30671

Reviewers: ddunbar, beanz, echristo, delcypher

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25496

llvm-svn: 285973