type_of metafunction should dealias the type

[katzdm]

P2996R6 will change type_of to no longer return reflections representing $typedef-name$s - instead, a reflection of the underlying type will be returned (see here).

The type_of function is implemented here, and we should also update the return_type_of metafunction (proposed by P3096), implemented here.

We have an existing desugarType utility in Metafunctions.cpp that often helps with tasks like these - there is an UnwrapAliases argument that should be set to true.

libcxx reflection tests should of course be updated to reflect this change.

[changkhothuychung]

Hi, its Nhat. picking this up.