Auto merge of #135281 - onur-ozkan:build-stamps, r=jieyouxu

centralize build stamp logic

This PR brings all the stamp file handling into one place inside `build_stamp` module, which takes care of everything related to build stamps. By doing this, we cut down on duplicated code and types and keep the codebase easier to maintain and more consistent.

Main goals are:

- Make stamp handling stricter so we don't have to pass `Path`s around and manually `join` on arbitrary directories
- Keep all stamp-related logic in one place
- Make it easier to test and debug
- Avoid duplication
- Keep things simple and well-documented

Resolves #134962

Author: bors

src/backend/debugging.md

@@ -56,7 +56,7 @@ These tools include:
 
 By default, the Rust build system does not check for changes to the LLVM source code or
 its build configuration settings. So, if you need to rebuild the LLVM that is linked
-into `rustc`, first delete the file `llvm-finished-building`, which should be located
+into `rustc`, first delete the file `.llvm-stamp`, which should be located
 in `build/<host-triple>/llvm/`.
 
 The default rustc compilation pipeline has multiple codegen units, which is