clang-format: Improve braced init list detection:

Before:
  std::this_thread::sleep_for(std::chrono::nanoseconds{
    std::chrono::seconds { 1 }
  } /
                              5);

After:
  std::this_thread::sleep_for(
      std::chrono::nanoseconds{ std::chrono::seconds{ 1 } } / 5);

This fixes llvm.org/PR16554.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189451 91177308-0d34-0410-b5e6-96231b3b80d8