Thread-safety checker updates.

Warn about calls from thread-safe code to methods of classes that have been marked as not_thread_safe. Previously, this would get an Unchecked warning, but these warnings are disabled in release builds because there are too many. This warning is more exact, so we should be able to leave it enabled.

While one can declare attributes on lambdas, the attributes attach to the (anonymous) type of the lambda, not to the function. This caused us to miss seeing not_thread_safe attributes on lambdas. Fixed.