clang-format: Basic support for C++1y.

Before:
  int bi{1 '000' 000};
After:
  int bi{1'000'000};

This fixes llvm.org/PR19342.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206263 91177308-0d34-0410-b5e6-96231b3b80d8