Auto merge of #51969 - pietroalbini:rollup, r=pietroalbini

Rollup of 7 pull requests

Successful merges:

 - #51511 (Stabilize Iterator::flatten in 1.29, fixes #48213.)
 - #51853 (Fix some doc links)
 - #51890 (Fix inconsequential typo in GlobalAlloc doc example)
 - #51920 (use literal span for concrete type suggestion)
 - #51921 (improve the error message when `#[panic_implementation]` is missing)
 - #51922 (rename the llvm-tools component to llvm-tools-preview and tweak its image)
 - #51961 (Fix typo in /src/librustc_resolve/lib.rs)

Failed merges:

r? @ghost