clang-format: Fix hanging nested blocks in macros.

Before:
  #define MACRO()                     \
    Debug(aaa, /* force line break */ \
          {                           \
      int i;                          \
      int j;                          \
          })

After:
  #define MACRO()                     \
    Debug(aaa, /* force line break */ \
          {                           \
            int i;                    \
            int j;                    \
          })

llvm-svn: 237108