[PlistSupport] Produce a newline to end plist output files

Summary:
As suggested in the review of D62949, this patch updates the plist
output to have a newline at the end of the file. This makes it so that
the plist output file qualifies as a POSIX text file, which increases
the consumability of the generated plist file in relation to various
tools.

Reviewers: NoQ, sfertile, xingxue, jasonliu, daltenty

Reviewed By: NoQ, xingxue

Subscribers: jsji, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63041

llvm-svn: 362992