Constant result ValueObjects are - well - constant

And they also do not have a thread/frame attached to them

That makes dynamic and synthetic values attached to them impossible to update - which, among other things, makes it impossible to properly display persistent variables of types that could have such dynamic/persistent values

Fix this by making it so that a ValueObject can control its constantness (hint: dynamic and synthetic values cannot be constant) and whether it wants to let itself be updated when an invalid thread is around

llvm-svn: 237504