Fixed a race condition in PrecompiledPreamble.
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

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@310618 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang