[Coroutines] Support opaque pointers in solveTypeName()

As far as I can tell, these names are only intended to be
informative, so just use a generic "PointerType" for opaque pointers.

The code in solveDIType() also treats pointers as basic types (and
does not try to encode the pointed-to type further), so I believe
this should be fine.

Differential Revision: https://reviews.llvm.org/D121280