[llvm] add zstd to `llvm::compression` namespace

- 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`

Reviewed By: leonardchan, MaskRay

Differential Revision: https://reviews.llvm.org/D128465