Remove pimpl class from PassRegistry.

Since removeRegistrationListener is no longer called during static
destruction, we can get rid of the pimpl in PassRegistry.

This should clean up the code somewhat, increase clarity, and also
allows us to put the Lock as a member of the class, instead of as a
ManagedStatic.

As part of this change, the PassInfo class is moved from
PassSupport.h to its own file, to eliminate the otherwise circular
header dependency between PassRegistry.h and PassSupport.h

Reviewed by: rnk, dblaikie

Differential Revision: http://reviews.llvm.org/D4107