[lit] Read command stdout/stderr as text on Windows

[lit] Read command stdout/stderr as text on Windows

This takes care of normalizing newlines back to single LF instead
of CRLF.

This on itself breaks on a couple tests that accidentally seem to
be writing binary data to stdout; make sure those cases are piped
to /dev/null instead of actually written to a terminal.

Differential Revision: https://reviews.llvm.org/D120623