[libclang] Protect against a race condition where a thread
may be destroying an ASTUnit while cleanupOnDiskMapAtExit is
getting called.

rdar://11781241

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159664 91177308-0d34-0410-b5e6-96231b3b80d8