[ADT] Mark BitVector::find_prev_unset const (NFC) (llvm#156272)

find_prev_unset calls find_last_unset_in, a const method, but
find_prev_unset itself isn't marked const.

Author: kazutakahirata

llvm/include/llvm/ADT/BitVector.h

@@ -327,7 +327,7 @@ class BitVector {
 
   /// find_prev_unset - Returns the index of the first unset bit that precedes
   /// the bit at \p PriorTo.  Returns -1 if all previous bits are set.
-  int find_prev_unset(unsigned PriorTo) {
+  int find_prev_unset(unsigned PriorTo) const {
     return find_last_unset_in(0, PriorTo);
   }