Revert "llvmPackages_15: update licenses"

This reverts commit 386aba3115176b11eb49a0606e9dd17506273776.

As I understand it from reading
<https://llvm.org/docs/DeveloperPolicy.html#copyright-license-and-patents>,
the structure of LLVM licensing is as follows:

 - They're in the process of relicensing to Apache-2.0 WITH LLVM-exception,
   but they haven't got permission to relicense all the code yet.
   This means that some of the code can be used under the new license,
   but not all of it, and it's difficult to know which is which.  This
   license is therefore probably not useful yet, until the relicensing
   effort is commit.

 - While the relicensing effort is ongoing, code being contributed to
   LLVM has to have permission to be used under the old and new
   licensing schemes.  Since the new licensing scheme can't be used
   for all code yet, it only makes sense to use LLVM's code under the
   old licensing scheme at the moment.

 - The old licensing scheme is that code for the LLVM components we
   care about is all available under the NCSA license, and some
   components are optionally available under a different license,
   usually the MIT license, instead.

So I think we should go back to just listing NCSA, or NCSA/MIT, and
forget about the new license until it actually becomes useful,
i.e. LLVM's relicensing effort is complete.

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