[randstruct] Don't allow implicit forward decl to stop struct randomization

If a struct/enum type used in a record doesn't have a forward decl /
def, an implicit one is injected into the struct. This stops clang from
randomizing the structure in some situations---i.e. when the struct
contains only function pointers. So we accept forward decls so they
don't prevent randomization.

Fixes 60349

Reviewed By: MaskRay, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D143300