clang-format: Fix infinite loop in macro special case.

If escaped newlines are aligned right
(FormatStyle.AlignEscapedNewlinesLeft == false), and a line contained
too many characters to fit into the column limit, this would result in
a (virtually) endless loop creating a negative number of spaces.

Instead, allow the escaped newlines to be pushed past the column limit
in this case.

This fixes llvm.org/PR16515.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189459 91177308-0d34-0410-b5e6-96231b3b80d8