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

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

- 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`
- debian users should install libzstd when using `LLVM_ENABLE_ZSTD=FORCE_ON` from source due to this bug https://bugs.launchpad.net/ubuntu/+source/libzstd/+bug/1941956

Reviewed By: leonardchan, MaskRay

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