Allow using uncompressed metadata for proc-macros and dylibs

[Aaron1011]

Currently, we always compress crate metadata for dylibs and proc-macros:

rust/compiler/rustc_interface/src/passes.rs

Lines 928 to 940 in ea7e131

	let metadata_kind = tcx
	.sess
	.crate_types()
	.iter()
	.map(|ty| match *ty {
	CrateType::Executable | CrateType::Staticlib | CrateType::Cdylib => MetadataKind::None,
	CrateType::Rlib => MetadataKind::Uncompressed,
	CrateType::Dylib | CrateType::ProcMacro => MetadataKind::Compressed,
	})
	.max()
	.unwrap_or(MetadataKind::None);

This prevents us from mmap-ing the metadata into memory when we decode it, since we need to read the entire file in order to decompress it.

We should allow configuring this setting via a -Z flag (and maybe through the Cargo.toml at some point), to allow users to trade disk space for speed. In addition to avoiding the overhead of compressing/decompressing, gaining the ability to mmap the crate metadata means that we may avoid loading unused portions from disk entirely. This is less important for proc-macros after #76897, but might still have a noticable performance impact for dylibs.

Suggested by @joshtriplett on Zulip.

[bjorn3]

I think it would make sense to unconditionally use uncompressed metadata for proc-macros. They aren't used at runtime anyway, so binary size isn't as important as for dylibs. Dylibs aren't as common and even for libstd.so we actually load the metadata from libstd.rlib instead so no decompression happens during builds unless someone builds a crate as rust dylib only and even then I suspect for regular cargo builds rustc will instead load the metadata from the .rmeta file instead.