Add new PrintingPolicy entry to trim number of newlines. Useful for the CFG printer.

The change isn't completely comprehensive.  This can be filled in
lazily as needed.  There is one consumer right now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197093 91177308-0d34-0410-b5e6-96231b3b80d8