[emacs] Add noext as an attribute in llvm-mode.el

[JonPsson1]

The NoExt attribute was introduced with #100757, to exist alongside with signext and zeroext.

Add "noext" as an attribute to llvm-mode.el to get the proper highlighting of the keyword.

(For test case using this attribute, see for example llvm/test/CodeGen/SystemZ/args-14.ll)

@goldsteinn @lukel97

[goldsteinn]

LGTM

let me know if you need me to merge for you.

[JonPsson1]

Thanks.

I tried to merge it here, but I got a mail:

"LLVM Premerge Checks: All jobs have failed"
https://github.com/llvm/llvm-project/actions/runs/12585196983
@goldsteinn

[goldsteinn]

Thanks.

I tried to merge it here, but I got a mail:

"LLVM Premerge Checks: All jobs have failed" https://github.com/llvm/llvm-project/actions/runs/12585196983 @goldsteinn

Don't worry about that. Change is in and I don't see it get reverted.