quotearg: don't attempt to store 1 << 31 into an "int"

quotearg: don't attempt to store 1 << 31 into an "int"

* lib/quotearg.c (quotearg_buffer_restyled): Building coreutils with
gcc's new -fsanitize=undefined and running its tests triggered some
new test failures due to undefined behavior, all with this diagnostic:
  lib/quotearg.c:629:62: runtime error: left shift of 1 by 31 places \
    cannot be represented in type int
Rather than shifting "1" left to form a mask, shift the bits right and
simply use "1" as the mask.
Co-authored-by: Paul Eggert <eggert@cs.ucla.edu>