Use SmallVector instead of std::vector to manage storage of llvm::BitVector

This is a follow-up to https://reviews.llvm.org/D100387.

std::vector is not the best storage container here. My local benchmark (counting
the number of instruction when compiling the sqlite3 amalgamation) yields the
following:

- std::vector<BitVector> -> 5,860,885,896
- SmallVector<BitWord, 0> -> 5,858,991,997
- SmallVector<BitWord> -> 5,817,679,224

Differential Revision: https://reviews.llvm.org/D100744