clang-format: Lower penalty for breaking between array subscripts.

Before:
  aaaaaaaaaaaaaaaaaaaaaaaaa[aaaaaaaaaaa(aaaaaaaaaaaa)][bbbbbbbbbbb(
      bbbbbbbbbbbb)]

After:
  aaaaaaaaaaaaaaaaaaaaaaaaa[aaaaaaaaaaa(aaaaaaaaaaaa)]
                           [bbbbbbbbbbb(bbbbbbbbbbbb)]

llvm-svn: 256343