Auto merge of #77372 - jonas-schievink:rollup-e5bdzga, r=jonas-schievink

Rollup of 12 pull requests

Successful merges:

 - #77037 (more tiny clippy cleanups)
 - #77233 (BTreeMap: keep an eye out on the size of the main components)
 - #77280 (Ensure that all LLVM components requested by tests are available on CI)
 - #77284 (library: Forward compiler-builtins "mem" feature)
 - #77296 (liveness: Use Option::None to represent absent live nodes)
 - #77322 (Add unstable book docs for `-Zunsound-mir-opts`)
 - #77328 (Use `rtassert!` instead of `assert!` from the child process after fork() in std::sys::unix::process::Command::spawn())
 - #77331 (Add test for async/await combined with const-generics.)
 - #77338 (Fix typo in alloc vec comment)
 - #77340 (Alloc vec use imported path)
 - #77345 (Add test for issue #74761)
 - #77348 (Update books)

Failed merges:

r? `@ghost`

src/doc/embedded-book

@@ -1 +1 @@
-Subproject commit 0cd2ca116274b915924c3a7e07c1e046b6f19b77
+Subproject commit dd310616308e01f6cf227f46347b744aa56b77d9

src/doc/rust-by-example

@@ -1 +1 @@
-Subproject commit 19f0a0372af497b34369cf182d9d16156cab2969
+Subproject commit 7d3ff1c12db08a847a57a054be4a7951ce532d2d

src/doc/unstable-book/src/compiler-flags/unsound-mir-opts.md

@@ -0,0 +1,8 @@
+# `unsound-mir-opts`
+
+--------------------
+
+The `-Zunsound-mir-opts` compiler flag enables [MIR optimization passes] which can cause unsound behavior.
+This flag should only be used by MIR optimization tests in the rustc test suite.
+
+[MIR optimization passes]: https://rustc-dev-guide.rust-lang.org/mir/optimizations.html