clang-format: Fix indentation of macros in include guards (after r312125).

Before:
  #ifndef A_H
  #define A_H

  #define A() \
  int i;    \
  int j;

  #endif // A_H

After:
  #ifndef A_H
  #define A_H

  #define A() \
    int i;    \
    int j;

  #endif // A_H

llvm-svn: 312484