Improve formatting of #defines.

Two improvements:
1) Always leave at least one space before "\". Otherwise is can look bad
   and there is a risk of unwillingly joining to characters to a different
   token.
2) Use the full column limit for single-line #defines.
   Fixes llvm.org/PR15148

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176245 91177308-0d34-0410-b5e6-96231b3b80d8