Do not ever allow using the full line in preprocessor directives.

We would format:
  #define A \
    int f(a); int i;
as
  #define A \
    int f(a);\
    int i

The fix will break up macro definitions that could fit a line, but hit
the last column; fixing that is more involved, though, as it requires
looking at the following line.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171715 91177308-0d34-0410-b5e6-96231b3b80d8