NOTE: wrok on this patch continues at https://reviews.llvm.org/D129786
I am continuing work on that revision because this one has a lot of outdated unrelated history.
- add `FindZSTD.cmake`
- add zstd to `llvm::compression` namespace
- add a CMake option `LLVM_ENABLE_ZSTD` with behavior mirroring that of `LLVM_ENABLE_ZLIB`
- add tests for zstd to `llvm/unittests/Support/CompressionTest.cpp`