Improve the vim code for highlighting trailing whitespace and lines

Improve the vim code for highlighting trailing whitespace and lines
longer than 80 columns. This replaces the heavy-handed "textwidth"
mechanism, and makes the trailing-whitespace highlighting lazy so
that it isn't constantly jumping on the user during typing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97267 91177308-0d34-0410-b5e6-96231b3b80d8