Add LLVM realtime sanitizer

[luca3s]

This is a new attempt at adding the LLVM real-time sanitizer to rust.

Previously this was attempted in rust-lang/rfcs#3766.

Since then the sanitize attribute was introduced in #142681 and it is a lot more flexible than the old no_santize attribute. This allows adding real-time sanitizer without the need for a new attribute, like it was proposed in the RFC. Because i only add a new value to a existing command line flag and to a attribute i don't think an MCP is necessary.

Currently real-time santizer is usable in rust code with the rtsan-standalone crate. This downloads or builds the sanitizer runtime and then links it into the rust binary.

The first commit adds support for more detailed sanitizer information.
The second commit then actually adds real-time sanitizer.
The third adds a warning against using real-time sanitizer with async functions, because it doesn't behave as expected when used with async functions. I am not sure if this is actually wanted, so i kept it in a seperate commit.
The fourth commit adds the documentation for real-time sanitizer.

[rustbot]

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

Some changes occurred in tests/ui/sanitizer

cc @rcvalle

Some changes occurred in src/tools/compiletest

cc @jieyouxu

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann

These commits modify compiler targets.
(See the Target Tier Policy.)

Some changes occurred in src/doc/unstable-book/src/compiler-flags/sanitizer.md

cc @rust-lang/project-exploit-mitigations, @rcvalle

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann

[rustbot]

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[jdonszelmann]

Looks ok from attr parsing pov :)

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- spec::tests::aarch64_apple_darwin stdout ----

thread 'spec::tests::aarch64_apple_darwin' (51697) panicked at compiler/rustc_target/src/spec/mod.rs:3010:9:
assertion `left == right` failed
  left: Err("supported-sanitizers[3]: unknown sanitizer realtime at line 1 column 1001")

[luca3s]

I have fixed the error above, but now i am getting a doc-test failure in compiler/rustc_lint_defs/src/builtin.rs, where i added the warning against using rtsan with async. The error message says that the usage of [sanitize(realtime = "...")] is invalid and it appears to not know about rtsan.

Do i need to bootstrap the doc test runner somehow, so that it knows about the attribute i just added?

[petrochenkov]

This change has a lang component (marking functions a blocking or non-blocking), so I'll send this to lang team first, before reviewing the implementation in detail.