[split-file] Respect input file's line endings

This change adds support for split-file to respect the line ending style
of the input file. This enables split-file to work as expected on
Windows with input files containing CRLF line endings.

The test files added along with this change mirror the existing basic
tests, but are forced to contain CRLF line endings via git attributes.
This will result in the tests always containing CRLF line endings when
checked out regardless of the user's OS.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D117897