Bug#935149: llvm-toolchain-9: New license, copyright file outdated

[Chris Hofstaedtler]

Package: llvm-toolchain-9
Version: 9~+rc2
Severity: important

Dear Maintainers,

LLVM has switched to Apache + LLVM Exceptions; the top-level NOTICE.txt
in the extracted llvm-toolchain-9-9~+rc2 source shows this.
However, it appears debian/copyright has not been updated to reflect
this.

As this has implications for GPLv2-only programs, I consider this to be
interesting, at the very least.

Thanks,
Chris