Using `ld.lld` may have been clever, but that was getting the /system/
ld.lld, not one we may have built as part of building llvm. By using the
warning message coming directly from rustc we now correctly skip the
zlib and zstd tests when the support is missing.
LLVM already supports emitting compressed debuginfo. In debuginfo=full
builds, the debug section is often a large amount of data, and it
typically compresses very well (3x is not unreasonable.) We add a new
knob to allow debuginfo to be compressed when the matching LLVM
functionality is present. Like clang, if a known-but-disabled
compression mechanism is requested, we disable compression and emit
uncompressed debuginfo sections.
The API is different enough on older LLVMs we just pretend the support
is missing on LLVM older than 16.
