when saving a module timestamp file

This commit doesn't include a test as it requires a test that reproduces
a file write/close error that couldn't really be constructed artificially.

rdar://31860650

Differential Revision: https://reviews.llvm.org/D33357


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304538 91177308-0d34-0410-b5e6-96231b3b80d8