[llvm] Use x.empty() instead of llvm::empty(x) (NFC)

I'm planning to deprecate and eventually remove llvm::empty.

I thought about replacing llvm::empty(x) with std::empty(x), but it
turns out that all uses can be converted to x.empty().  That is, no
use requires the ability of std::empty to accept C arrays and
std::initializer_list.

Differential Revision: https://reviews.llvm.org/D133677