[llvm] [TableGen][Docs] Accept "code" as a Type (PR #124902)

[Jay Foad via llvm-commits]

jayfoad wrote:

> However the parser implementation accepts "code" as a Type with only one
> place where it is not allowed, corresponding to this production:
> 
> SimpleValue9: BangOperator ["<" Type ">"] "(" ValueListNE ")"

TBH I don't see the need for this restriction. Why don't we just allow e.g. `!cast<code>(...)` with `code` being an alias for `string`, just like it is everywhere else?

https://github.com/llvm/llvm-project/pull/124902