[index/store] Put the USR validation checking logging behind an environment variable
[index/store] Put the USR validation checking logging behind an environment variable

rdar://30643167

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