Add LICENSE.TXT covering contributions made by ARM.

Absent a Contributor's License Agreement (CLA) with an LLVM legal entity and as
reviewed and agreed with Chris Lattner, add a patent license covering future
contributions from ARM until there is a CLA. This is to make explicit ARM's
grant of patent rights to recipients of LLVM containing ARM-contributed
material.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171721 91177308-0d34-0410-b5e6-96231b3b80d8