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

Author: serge-sans-paille

llvm/include/llvm/ADT/BitVector.h

@@ -79,7 +79,7 @@ class BitVector {
   static_assert(BITWORD_SIZE == 64 || BITWORD_SIZE == 32,
                 "Unsupported word size");
 
-  using Storage = std::vector<BitWord>;
+  using Storage = SmallVector<BitWord>;
 
   Storage Bits;  // Actual bits.
   unsigned Size; // Size of bitvector in bits.