Format strings: don't ever convert %+d to %lu.

Presumably, if the printf format has the sign explicitly requested, the user
wants to treat the data as signed.

This is a fix-up for r172739, and also includes several test changes that
didn't make it into that commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172762 91177308-0d34-0410-b5e6-96231b3b80d8