[llvm] [TableGen] Add let append/prepend syntax for field concatenation (PR #182382)

[Mehdi Amini via llvm-commits]

https://github.com/joker-eph approved this pull request.

Awesome :) 

LGTM, but let's wait for another volunteer to review if possible.

https://github.com/llvm/llvm-project/pull/182382