Fix handling of comments in macros.

Fix handling of comments in macros.

We now correctly format:
 // Written as a macro, it is reformatted from:
 #define foo(a)                                                                \
   do {                                                                        \
     /* Initialize num to zero. */                                             \
     int num = 10;                                                             \
     /* This line ensures a is never zero. */                                  \
     int i = a == 0 ? 1 : a;                                                   \
     i = num / i; /* This division is OK. */                                   \
     return i;                                                                 \
   } while (false)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174517 91177308-0d34-0410-b5e6-96231b3b80d8