Avoid a potential race between stat() and open() of ASTFile

We need to open an ASTFile while checking its expected size and
modification time, or another clang instance can modify the file between
the stat() and the open().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207735 91177308-0d34-0410-b5e6-96231b3b80d8