clang-format: Fix regression introduced by r189337.

Before:
  if (aaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaa(
          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa) == 5) ...

After:
  if (aaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaa(
          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa)
      == 5) ...

Also precompute startsBinaryExpression() to improve performance.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190124 91177308-0d34-0410-b5e6-96231b3b80d8