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

[ckissane]

Summary

• 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
• add a cmake macro (TryFindDependencyMacro.cmake) to aid in finding "modern cmake" style dependencies such as Zstd in an "optional" or "REQUIRED" manner.

[ckissane]

• add_library Zstd::zstd in FindZSTD.cmake

[ckissane]

• simplify zstd cmake code (by using "modern cmake" and no need for FindZSTD anymore)

[ckissane]

• make llvm-config --system-libs support output reflect zstd usage

[ckissane]

• remove get_library_name zstd because non standard

[sebastian-ne]

It seems like FindZSTD.cmake went missing in the Diff 3 revision of this patch?

[ckissane]

In D129786#3654396, @sebastian-ne wrote:

It seems like FindZSTD.cmake went missing in the Diff 3 revision of this patch?

@sebastian-n yes because facebook is surprisingly good at "modern cmake" practices with shipping libzstd and so `find_dependency(zstd)` is enough to have the lib `zstd::libzstd_shared``` get defined!
I have hopes that this will fix the issues mac os users reported with the old version of this patch.

[ckissane]

make it try to find dependency instead of normal find_dependency macro (because the return breaks things on failure)

[sebastian-ne]

In D129786#3655910, @ckissane wrote:

In D129786#3654396, @sebastian-ne wrote:

It seems like FindZSTD.cmake went missing in the Diff 3 revision of this patch?

@sebastian-n yes because facebook is surprisingly good at "modern cmake" practices with shipping libzstd and so `find_dependency(zstd)` is enough to have the lib `zstd::libzstd_shared``` get defined!
I have hopes that this will fix the issues mac os users reported with the old version of this patch.

Nice, makes sense. It definitely fixes the issue when llvm is included with add_subdirectory.

llvm/cmake/modules/LLVMConfig.cmake.in

81

Should this be lower case zstd? I saw some cmake errors here and they disappear when this is lower case.

[ckissane]

• move try_find_dependency into it's own file

[ckissane]

llvm/cmake/modules/LLVMConfig.cmake.in

81

good catch, It should also probably be another find_dependency call instead of find_package so I fixed that too

[ckissane]

• add newline to end of TryFindDependencyMacro.cmake

[ckissane]

• clarify comment in TryFindDependencyMacro.cmake

[ckissane]

Abandoned because it duplicates https://reviews.llvm.org/D128465