Fixed a race condition in PrecompiledPreamble.

Summary:
Two PrecompiledPreambles, used in parallel on separate threads,
could be writing preamble to the same temporary file.

Reviewers: bkramer, krasimir, klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36529

llvm-svn: 310618