Add descriptions to the github issue labels

[RKSimon]

Many of the issue labels are self explanatory, but others are vague:

e.g. build-problem
Is this for problems building llvm/clang or building WITH llvm/clang - we have both types of ticket filed under there

Other examples I noticed (but there's probably others that are confusing to some): code-cleanup / code-quality / performance

[llvmbot]

@llvm/issue-subscribers-infrastructure

[EugeneZelenko]

I think will be good idea if code owners will participate in adding descriptions, so please ping them in this ticket.