Simplify BitVector code

Authored by serge-sans-paille on Tue, Apr 13, 7:14 AM.


Simplify BitVector code

Instead of managing memory by hand, delegate it to std::vector. This makes the
code much simpler, and also avoids repeatedly computing the storage size.

According to valgrind --tool=callgrind, this also slightly decreases the
instruction count, but by a small margin.

Differential Revision: https://reviews.llvm.org/D100387