Move to LLVM's license, delete LLVMBuild.txt files

[dpaoliello]

• Update header comments and LICENSE files to match LLVM ("Apache License v2.0 with LLVM Exceptions").
• Add missing header comments.
• Delete vestigial LLVMBuild.txt files (LLVMBuild was removed from LLVM a long time ago).

Fixes #87

[vtjnash]

There may need to be some work to confirm that all contributors are okay with the change here. I am unaware of the current status of the equivalent effort for LLVM.

[hikari-no-yume]

The old and new licenses are compatible, so it's essentially sublicensing rather than relicensing and should be no issue (#87).

[dpaoliello]

There may need to be some work to confirm that all contributors are okay with the change here. I am unaware of the current status of the equivalent effort for LLVM.

LLVM is still in the process of relicensing their source: https://foundation.llvm.org/docs/relicensing/

The old and new licenses are compatible, so it's essentially sublicensing rather than relicensing and should be no issue (#87).

Correct - per the link above the legacy license is kept for parts that haven't been relicensed yet and "Once the codebase is fully covered by the new license, we’ll drop the old license."

[vtjnash]

I don't see how sublicensing is relevant here. Sublicensing is your right to ship this software to someone else under the terms of the original LLVM Release License. It does not give you the right to change or remove any text of the original license or to claim the copyright yourself.

[dpaoliello]

I don't see how sublicensing is relevant here. Sublicensing is your right to ship this software to someone else under the terms of the original LLVM Release License. It does not give you the right to change or remove any text of the original license or to claim the copyright yourself.

Fair point.

It is relicensing, but the inclusion of the "Legacy License" allows the old code to be licensed under that until those previous contributors can agree to relicense.

[vtjnash]

That is true. Also, the list of contributors here is much smaller though. Someone could go back through the git log from LLVM, generate a list of authors, and check that much smaller list against the master list from LLVM and determine if this is already valid to change now.

[vtjnash]

On hold waiting for someone to do an authorship listing and contact the LLVM foundation for confirmation