Fixes #78398

I forgot to handle this case in #77255

The checksum cache allows to reuse the calculated checksums between
build-manifest and promote-release, or between multiple invocations of
build-manifest.

This lets rustc users tweak whether all functions should be put in their own
TEXT section, using whatever default value the target defines if the flag
is missing.

Uplift `temporary-cstring-as-ptr` lint from `clippy` into rustc

The general consensus seems to be that this lint covers a common enough mistake to warrant inclusion in rustc.
The diagnostic message might need some tweaking, as I'm not sure the use of second-person perspective matches the rest of rustc, but I'd like to hear others' thoughts on that.

(cc #53224).

r? @oli-obk

check object safety of generic constants

As `Self` can only be effectively used in constants with `const_evaluatable_checked` this should not matter outside of it.

Implements the first item of #72219

> Object safety interactions with constants

r? @oli-obk for now cc @nikomatsakis

libc: 0.2.79 -> 0.2.80

This PR bumps the version of the libc crate from 0.2.79 to 0.2.80 in order to hopefully fix a build failure when building the standard library for the tier 3 `x86_64-unknown-dragonfly` target.

Remove tokens from foreign items in `TokenStripper`

Fixes #78398

I forgot to handle this case in #77255

…r=Mark-Simulacrum

Add checksums cache to build-manifest

During the release process we're currently calculating the SHA256 of each file three times:

1. In `build-manifest`, to fill the `hash = "f00"` keys of the manifests.
2. In `promote-release`, to generate the `.sha256` files.
3. In `promote-release`, to generate the `.asc` GPG signatures.

Calculations 1. and 2. could be merged into a single one if there was a way for `build-manifest` to pass the checksums it generated over to `promote-release`. Unfortunately calculation 3. can't be merged as GPG requires extra metadata to be hashed.

This PR adds support for merging 1. and 2. by creating the `BUILD_MANIFEST_CHECKSUM_CACHE` environment variable, which points to a JSON file storing a cache of all the calculated checksums. `build-manifest` will load it at startup and avoid generating existing checksums, and it will dump its internal checksums cache into it when it exits successfully.

This PR also allows to run `build-manifest` multiple times without the need to wait for checksums to be calculated in the following invocations. The speedup will allow to work torwards a fix for rust-lang/promote-release#15 without impacting the release process duration nor our storage costs.

This PR can be reviewed commit-by-commit.
r? @Mark-Simulacrum

Implement -Z function-sections=yes|no

This lets rustc users tweak whether all functions should be put in their own TEXT section, using whatever default value the target defines if the flag is missing.

I'm having fun experimenting with musl libc and trying to implement the start symbol in Rust, that means avoiding code that requires relocations, and AFAIK putting everything in its own section makes the toolchain generate `GOTPCREL` relocations for symbols that could use plain old PC-relative addressing (at least on `x86_64`) if they were all in the same section.

rustc_span: avoid hashing ExpnId tag when using cached hash

Prefer new associated numeric consts in float error messages

CC #78382

Fix typo in  comment

I hope I got all the typos in that word. :)