clang-format: Fix column limit violation for merged lines in macros.

Before (81 columns):
  #define A                                                                       \
    void aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() { return aaaaaaaa; } \
    int i;

After:
  #define A                                                    \
    void aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() { \
      return aaaaaaaa;                                         \
    }                                                          \
    int i;

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@200974 91177308-0d34-0410-b5e6-96231b3b80d8