clang-format: Properly handle braced lists in macros.

Before:
  #define A    \
      { a, a } \
      ,

After:
  #define A {a, a},

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245837 91177308-0d34-0410-b5e6-96231b3b80d8