[llvm] [NFC][TableGen] DirectiveEmitter code cleanup (PR #107775)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

Thank you for the cleanup.  Now, is there any way you could separate the `{}->()` conversions with the rest?  A `{}->()` conversion can have a surprising effect like `std::vector<int> Vec(3,5)` vs `std::vector Vec{3, 5}`, so I would have to be more careful and understand each type's constructor than, say, when reviewing comments or removal of `llvm::`.

https://github.com/llvm/llvm-project/pull/107775