Add a new bit-vector implementation optimized for

IRGen's needs.

Swift SVN r24295

utils/test-clustered-bit-vector/test.cpp

@@ -0,0 +1,9 @@
+#include "swift/Basic/ClusteredBitVector.h"
+using namespace swift;
+int main() {
+  ClusteredBitVector cbvs[16];
+  cbvs[7].appendSetBits(65);
+  cbvs[3].appendClearBits(118);
+  cbvs[7] = std::move(cbvs[3]);
+  assert(cbvs[7][64] == false);
+}