Auto merge of #59303 - euclio:remove-rebuild-trigger, r=cuviper
replace llvm-rebuild-trigger with submodule commit hash
As mentioned in #59285.
This PR removes the need to update the `llvm-rebuild-trigger` file. Instead, the latest commit hash of the appropriate LLVM submodule will be stored in the stamp file and used to detect if a build is required.
Fixes #42405.
Fixes #54959.
Fixes #55537.