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patch continues at https://reviews.llvm.org/D129786 I am continuing work on that revision because this one has a lot of outdated unrelated history.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`

NOTE: work 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patch continues at https://reviews.llvm.org/D129786 I am continuing work on that revision because this one has a lot of outdated unrelated history.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`

NOTE: wrokork 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patch continues at https://reviews.llvm.org/D129786 I am continuing work on that revision because this one has a lot of outdated unrelated history.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`