clang-format: Improve expression heuristics.

Upon encountering a binary operator inside parentheses, assume that the
parentheses contain an expression.

Before:
  MACRO('0' <= c&& c <= '9');

After:
  MACRO('0' <= c && c <= '9');

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212040 91177308-0d34-0410-b5e6-96231b3b80d8