Codify the use of Apache License 2.0 software.

Subversion (svnlite) has always been Apache-licensed and LLVM
will be as well starting with version 9.0.0.
Note that core@ must still approve imports of new Apache-license
components in the tree as well as license changes to Apache ones.

Thanks to Brooks for the writeup.

Approved by: core@
Differential Revision: https://reviews.freebsd.org/D19358