[llvm] [TableGen] Support automatic type deduction (PR #109434)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I'm not a big fan of auto in general, but for tablegen I think it will only make it more difficult to read. It's already hard enough to figure out what's going on without looking at the fully expanded form. 

https://github.com/llvm/llvm-project/pull/109434