r193058 - Implement function type checker for the undefined behavior sanitizer.

[Tim Northover]

Hi Peter,

> This uses function prefix data to store function type information at the
> function pointer.

I'm not sure if this is a valid point, but should we add "function" to
allowedOpt in SanitizerArgs.cpp? Currently giving -fsanitize=function
to the driver gives an error.

Or is it hidden for now while we make sure it works?

Cheers.

Tim.