devel/llvm15: add a lite FLAVOR

The lite flavor is for when you just need a FreeBSD compiler (e.g., in
CI). By default it includes CLANG and LLD as well as backends required
for FreeBSD and FreeBSD ports (BE_NATIVE, BE_AMDGPU, and BE_WASM).

The lite install is ~350GiB on amd64 vs ~1.7GiB for default and it has
only a single dependency (zstandard). (The goal was no dependencies,
but libzstd is a private lib in base.)

Bump PORT_REVISION due to removal of clang libxml2 dependency.

Sponsored by: DARPA
(cherry picked from commit af979dd126ff49eabb75e48087ee284e288da322)