[NFC][TableGen] DirectiveEmitter code cleanup

[jurahul]

Eliminate unnecessary llvm:: prefix as this code is in llvm namespace.
Use ArrayRef<> instead of std::vector references when appropriate.
Use .empty() instead of .size() == 0.

[kazutakahirata]

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::.

[jurahul]

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::.

I may be, though all the {} -> () changes are for constructors with single arguments here, and for classes defined in the DirectiveEmitter.h, so the risk of the issues you mentioned above is less. But I can if you insist.

[jurahul]

Ok, I removed the {} -> () change. @kazutakahirata PTAL.

[kazutakahirata]

LGTM. Thanks!