[llvm][support] Replace `std::vector<bool>` use in YAMLTraits

LLVM Programmer’s Manual strongly discourages the use of `std::vector<bool>` and suggests `llvm::BitVector` as a possible replacement.

This patch replaces the use of `std::vector` with `llvm::BitVector` in LLVM's YAML traits and replaces the call to `Vec.insert(Vec.begin(), N, false)` on empty `Vec` with `Vec.resize(N)`, which has the same semantics but avoids using `insert` and iterators, which `llvm::BitVector` doesn't possess.

Reviewed By: dexonsmith, dblaikie

Differential Revision: https://reviews.llvm.org/D118111