[Clang] Convert some tests to opaque pointers (NFC)

[Clang] Convert some tests to opaque pointers (NFC)

These tests all require some adjustments to make sure that struct
types still get generated, mostly done by stripping pointer
indirections.

Some of this may no longer test the situation it was originally
intended for, e.g. the issue from pr18962 just doesn't really
exist anymore with opaque pointers, as we no longer generate
recursive types.