[IndexDataStore] Sort unit names using std::sort()
[IndexDataStore] Sort unit names using std::sort()

std::string is not POD.
(cherry picked from commit dcd292d511866c4fe33a90c6a37b9cf640458216)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang