Consider making cfg(feature = "crt-static") available *only* for link-time configuration

[petrochenkov]

cfg predicates used for link-time configuration (#[link(cfg(predicate))] #37406 #72059) may want to use data that is available only long after the regular cfg expansion happens.

For example,

• whether we are linking an executable or library (that's the case for crt-static)
• whether we use dynamic linking (in some sense) (that's the case for windows-gnu target compatibility with toolchains providing LLVM tools only #72241 (comment))

We can come up with some predicate value during regular cfg expansion, but it may be different from the predicate value that we really want at link time.

Is cfg(feature = "crt-static") ever used for regular cfg expansion?
It was created mostly with link-time configuration in mind.
We probably need to check it with crater.

[retep998]

Do people even use cfg(feature = "crt-static")? The only time I can think of is either in libc or in build scripts to decide how to build C/C++ code (in which case they pull it from environment variables and not cfg directly).

[petrochenkov]

@retep998
I wouldn't expect such use, but who knows, we need to check at least.

[petrochenkov]

So, right now the value of crt-static predicate is evaluated during regular cfg expansion, even if it's used during link-time configuration.
After #69519 (which introduced separate crt-static defaults for executables and libraries basically) this scheme is incorrect, and means that proc macro crates can get linked with static libc anyway in some cases.

So in addition to deprecating expansion time use of this predicate we also need to move its actual evaluation to link time where the crate type is known.
(To clarify, both "expansion time" and "link time" are about building the final binary, #[link(cfg)] is still lazy.)

[petrochenkov]

Unfortunately, cfgs in #[link] attributes also need to be evaluated before linking during codegen to determine library kinds.

One query using them is is_dllimport_foreign_item used to set dllimport attributes.
Another one is is_statically_included_foreign_item used to determine reachable non-generic items for symbol exports (reachable_non_generics_provider).