devel/llvm13 port ?

Hi,

While there is on-going work to get llvm13 in base, I started to look
to make a devel/llvm13 port, independant from devel/llvm (which is
mirroring version in base), in order to be able to continue working on
recent compilers which require recent llvm version.

For what I am aware:
- lang/rust : the next version (in 6 weeks) require at least llvm12 (and I don't
even build it for now due to this missing dependency)
- lang/zig : the next version (before end of month) require llvm13

And I want to avoid building embedded llvm13 in both ports.

The attached port is based on mortimer@ work instead of plain llvm
version. It permits to avoid patching llvm and reuse the (good) work
already done instead of duplicate it.

To avoid conflict with devel/llvm, I installed devel/llvm13 inside
${LOCALBASE}/llvm13 . It should make it undetectable by others ports,
and shouldn't influence them.

I intent to not link it in the build (devel/Makefile) and let's ports
using it in BUILD_DEPENDS to trigger the build of the port. It should
avoid building it on archs where no others ports are using it (Am I
right ?)

The devel/llvm13 port is simple and focus on providing required parts
for rust or zig: llvm libraries, clang binary and libraries, lld
binary and libraries. It doesn't include lldb or python module for
example.

Once devel/llvm will reach llvm-13 version, this port could be pushed
to Attic. It should permit in mid-term to continue to have lang/rust
or lang/zig working without pushing too much pressure on
llvm13-in-base work.

MAINTAINER variable is undefined for now, but I would be fine to take
the maintainership of it (as I am the main customer for it with rust
or zig).

Comments or OK would be welcome.
--
Sebastien Marie