Cross-compiling libLLVM requires version-matched llvm

[Taylor R Campbell <campbell+netbsd-tech-pkg%mumble.net@localhost>]

Cross-compiling X these days requires Mesa which requires libLLVM.

libLLVM in turn requires running llvm-tblgen.  It can build
llvm-tblgen internally for native builds, but it has no mechanism like
CXX_FOR_BUILD for cross builds, for which it relies on a preinstalled
llvm-tblgen, which comes from llvm.

However, libLLVM requires a version-matched llvm for llvm-tblgen, and
there is version skew between lang/libLLVM (13.0.1) and lang/llvm
(15.0.7).  What to do?

1. Reimport llvm 13.0.1 as lang/llvm13 verbatim.  Problem: conflicts
   with lang/llvm.

2. Reimport llvm 13.0.1 as lang/llvm13, but put under a different
   prefix like ${PREFIX}/llvm13.

3. See if we can get by with a single version.  Problem: sounds
   nontrivial; presumably there was a reason for the stern comment in
   lang/llvm/Makefile about not updating libLLVM at the same time.