clang-format: Don't corrupt macros with open braces.

Formatting:
  #define A { {
  #define B } }

Before:
  #define A               \
    {                     \
      { #define B }       \
    }

After:
  #define A               \
    {                     \
      {
  #define B               \
    }                     \
    }

This fixes llvm.org/PR22884.

llvm-svn: 232166