Add LICENSE

[jdorfman]

Test plan

N/A

[varungandhi-src]

Could you clarify why this is needed? Separate directories contain their own licenses and individual files contain license headers. For example: https://github.com/sourcegraph/lsif-clang/blob/main/clang/LICENSE.TXT

[jdorfman]

@varungandhi-src Good question. That license belongs to the LLVM not Sourcegraph. The license for our project should be in the root directory so it is clear for others that this project has an OSI-approved license. It helps with adoption. Once this is merged it will display this:

[varungandhi-src]

This project started out as a fork of LLVM, so I think the easiest path here is to use the same license as the original project (which is essentially OSI-approved, it only spells out more freedoms explicitly). Assigning a slightly different license only to the code that we have authored seems tricky, especially given that the history of this repo has been truncated.

I suggest a couple of possible options (don't need to be exclusive):

• Document in the README that lsif-clang is distributed under the same license as LLVM, which is Apache 2.0 with LLVM Exceptions.
• Symlink clang-tools-extra/LICENSE.txt from the root if there is no other way to populate the license field in the sidebar (it looks like GitHub ignores it if it is under .github/), because the lsif-clang folder lives under clang-tools-extra/. (I know symlinking works for the README file.)

Adding a different license at the root of the repo is likely to cause unnecessary confusion IMO because it becomes unclear what files it applies to.

If GitHub still refuses to update the sidebar, then so be it. I think "correctness" matters more here; I doubt that any serious person will not use lsif-clang because GitHub's UI is not showing the license in the sidebar.