rustc_attr::Stability inlined a lot of dublicates in rustc dependency crates rmeta's, increasing it's size

[klensy]

Look at this place: this Stability value looks like const, but still inserted multiple times into *.rmeta file.

rust/compiler/rustc_passes/src/stability.rs

Lines 716 to 728 in 9ccf661

	if tcx.sess.opts.debugging_opts.force_unstable_if_unmarked {
	let reason = "this crate is being loaded from the sysroot, an \
	unstable location; did you mean to load this crate \
	from crates.io via `Cargo.toml` instead?";
	let stability = tcx.intern_stability(Stability {
	level: attr::StabilityLevel::Unstable {
	reason: Some(Symbol::intern(reason)),
	issue: NonZeroU32::new(27812),
	is_soft: false,
	},
	feature: sym::rustc_private,
	});
	annotator.parent_stab = Some(stability);

For example, text from reason (and other fields from Stability):
found 172 times in libaddr2line.rmeta
found 911 times in libhashbrown.rmeta
found 9590 times in libobject.rmeta

rustc 1.56.0-nightly (5a19ffe1c 2021-08-13)
binary: rustc
commit-hash: 5a19ffe1c2b99d9e09706cc286aad1ec0868eddb
commit-date: 2021-08-13
host: x86_64-pc-windows-msvc
release: 1.56.0-nightly
LLVM version: 12.0.1

@rustbot label: +I-heavy

[klensy]

@rustbot label: +A-metadata +C-enhancement