Auto merge of #121579 - RalfJung:miri, r=RalfJung

Miri subtree update

r? `@ghost`

README.md

@@ -279,7 +279,7 @@ Miri builds and vice-versa.
 
 You may be running `cargo miri` with a different compiler version than the one
 used to build the custom libstd that Miri uses, and Miri failed to detect that.
-Try deleting `~/.cache/miri`.
+Try running `cargo miri clean`.
 
 #### "no mir for `std::rt::lang_start_internal`"
 
@@ -465,7 +465,7 @@ Moreover, Miri recognizes some environment variables:
   must point to the `library` subdirectory of a `rust-lang/rust` repository
   checkout. Note that changing files in that directory does not automatically
   trigger a re-build of the standard library; you have to clear the Miri build
-  cache manually (on Linux, `rm -rf ~/.cache/miri`;
+  cache with `cargo miri clean` or deleting it manually (on Linux, `rm -rf ~/.cache/miri`;
   on Windows, `rmdir /S "%LOCALAPPDATA%\rust-lang\miri\cache"`;
   and on macOS, `rm -rf ~/Library/Caches/org.rust-lang.miri`).
 * `MIRI_SYSROOT` (recognized by `cargo miri` and the Miri driver) indicates the sysroot to use. When