[llvm] added a script to update llvm-mc test file (PR #107246)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson commented:

This looks really useful! I wonder if we could also make it work for the error message checking?

E.g. generate the following check lines
`CHECK-ERR: [[#@LINE]]:<col>: error: ...`

if the RUN: line contains something like `not llvm-mc .... 2>&1 | FileCheck`?

https://github.com/llvm/llvm-project/pull/107246