Better output for long help strings for command-line options.

Summary:
This patch allows using \n inside long help strings for command-line
options, so that all lines are equally indented. This is not a perfect solution,
as we don't (and probably don't want to) know about terminal width, but it
allows to format long help strings somehow readable without manually padding
them with spaces. A motivating example is -help output from clang-format (source
code in tools/clang-format/ClangFormat.cpp, see cl options offset, length,
style, and dump-config).

Reviewers: atrick, alexfh

Reviewed By: alexfh

CC: llvm-commits, rafael

Differential Revision: http://llvm-reviews.chandlerc.com/D779