Use backslash escape, replacing xargs -0 in test macro-multiline.c

Summary:
xargs supports escaping of newline characters with backslash.
xargs -0 is neither part of POSIX nor the LSB.

This patch removes the -0 option and adjusts the input to xargs
accordingly; that is, the input is a text file not ending in an
incomplete line, and the newline of interest is preceded by a backslash.

Note: The treatment of escaped newline characters is not as clearly
specified by POSIX as for escaped blank characters; however, the same
can be said for escaped backslashes. It is slightly more clear for the
case where the -I option is used; however, -I is also of limited
portability.

Reviewers: bruno

Reviewed By: bruno

Subscribers: bruno, rcraik, cfe-commits

Differential Revision: https://reviews.llvm.org/D41544

llvm-svn: 321828