Merging rr302639:

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r302639 | azanella | 2017-05-10 13:18:25 +0100 (Wed, 10 May 2017) | 11 lines

[msan] Fix getmntent{_r} for empty /etc/fstab

Some configuration (for instance default docker ubuntu images) uses
a default empty and invalid /etc/fstab configuration file.  It makes
any call to getmntent return NULL and it leads to failures on
Msan-aarch64{-with-call}-Test/MemorySanitizer.getmntent{_r}.

This patch fixes it by creating a temporary file with some valid
entries (although not valid for the system) to use along with
setmntent/getmntent.

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llvm-svn: 303146