[clang] [llvm] [MC,clang,llvm-readobj,yaml2obj] Support CREL relocation format (PR #91280)

[Peter Smith via cfe-commits]

smithp35 wrote:

I will take a look, but may take me a while to go through it. Some very quick comments:
* Would `--enable-experimental-crel` be a better name than `--experimental-crel` that makes it clear that the option is just an unlock of another flag, and not a flag in of itself?
* I agree that the section type should be in the generic range. While it is in the experimental stage it could be possible to put it in  SHT_LOOS SHT_HIOS range temporarily. When there is an official ELF code in the generic range it could be changed (forcing a recompile, or a migration script), or just have the LLVM tools implicitly convert from the old code to the new. For the experimental range.
* While I can give a personal opinion. This will touch multiple components of LLVM each with their own code-owners. Will probably need a signal, either on the review or via discourse threads that the community is happy to go ahead with the addition of CREL. 

https://github.com/llvm/llvm-project/pull/91280