[llvm] cmake config groundwork to have ZSTD in LLVM

- added `FindZSTD.cmake`
- added a CMake option `LLVM_ENABLE_ZSTD` with behavior mirroring that of `LLVM_ENABLE_ZLIB`
- likewise added have_zstd to compiler-rt/test/lit.common.cfg.py, clang-tools-extra/clangd/test/lit.cfg.py, and several lit.site.cfg.py.in files mirroring have_zlib behavior

Reviewed By: leonardchan, MaskRay

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

llvm/lib/Support/CMakeLists.txt

@@ -25,6 +25,10 @@ if(LLVM_ENABLE_ZLIB)
   set(imported_libs ZLIB::ZLIB)
 endif()
 
+if(LLVM_ENABLE_ZSTD)
+  list(APPEND imported_libs zstd)
+endif()
+
 if( MSVC OR MINGW )
   # libuuid required for FOLDERID_Profile usage in lib/Support/Windows/Path.inc.
   # advapi32 required for CryptAcquireContextW in lib/Support/Windows/Path.inc.