ADT/STLExtras: Introduce llvm::empty; NFC

This is modeled after C++17 std::empty().

Differential Revision: https://reviews.llvm.org/D53909

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345679 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed