[llvm-objcopy] Support --decompress-debug-sections when zlib is disabled

When zlib is disabled at build time, the diagnostic `LLVM was not compiled with
LLVM_ENABLE_ZLIB: cannot decompress` for --decompress-debug-sections may be
inaccurate: if zstd is enabled, we should still support zstd decompression.

It's not useful to test zlib and zstd. Just remove the diagnostic and add a new
one before `compression::decompress`.

This fixes compress-debug-sections-zstd.test

Reviewed By: mariusz-sikora-at-amd, jhenderson, phosek

Differential Revision: https://reviews.llvm.org/D135744