[libclang] Check for the 'LIBCLANG_NOTHREADS' environment variable before creating a separate thread for code-completion.
[libclang] Check for the 'LIBCLANG_NOTHREADS' environment variable before creating
a separate thread for code-completion.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@170978 91177308-0d34-0410-b5e6-96231b3b80d8

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