[SUA] Add test for allocation of classes without metadata (#63759)
Add test for allocating classes with pruned metadata and refactor
`computeMallocTypeSummary()` to make it easier to understand:
* Use early returns for error (metadata absent) conditions
* Remove reliance on implicit dependency---having a type descriptor
currently implies that there is also class metadata---in case this
ever changes
Co-authored-by: Julian Lettner <julian.lettner@apple.com>