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

[serge-sans-paille]

Summary

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 amalgametion) yields the following:

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

[thakis]

Thanks!

llvm/include/llvm/ADT/BitVector.h

 class BitVector {
   typedef uintptr_t BitWord;
 
   enum { BITWORD_SIZE = (unsigned)sizeof(BitWord) * CHAR_BIT };
 
   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.
 
 public:
   typedef unsigned size_type;
 
   // Encapsulation of a single bit.