Add new headers files (terminating with `BB`, as `BasicBlockNodeBB.h`),
which will be used as the new header files by the users, and which
contain a declaration of the template instantiation, and a `using`
aliasing the instantiation.
This has been made to comply with the `-Wundefined-func-template` flag,
automatically enabled when using `-Weverything`
`BasicBlockNode` and `RegionCFG` classes are now template classes. This
means that the `BasicBlockNode` class can be used as a generic wrapper
for any type of object in the original graph (it is usually used to wrap
a `llvm::BasicBlock *` for decompilation purposes, but in tests it can
be used to wrap a `DotNode` object) that implementes `GraphTraits`.
When creating the collapsed node, remember to add in the substitution
phase of the old nodes of the regions (relative to the outer regions),
also any nodes added as default `SetNode`s for any entry dispatcher
inserted.
Pietro Fezzardi