question about license for clang intrinsic headers...

I was about to copy wmmintrin.h for the FreeBSD project, but I noticed
that the file (clang/lib/Headers/wmmintrin.h) does not have a copyright
statement in it, though it says:
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.

Also, the license is significantly different than what is contained in
LICENSE.TXT, as the LICENSE.TXT has the three clauses, but the file does
not have the three clauses...

What shall I do to properly attribute the wmmintrin.h file to the LLVM

Thank you.

Attribution of that file is not required. It is licensed under the MIT license, described here: