According to the C++ standard, `dynamic_cast` of pointers either returns
a pointer (7.6.1.7) or results in undefined behavior (11.9.5). This
patch marks `__dynamic_cast` as `willreturn` to remove unused calls.
Fixes#77606.
`__dynamic_cast` relies on `type_info`, which its signature assumed to be in the generic / default address space. This patch corrects the oversight (we know that `type_info` resides in the GlobalVar address space) and adds an associated test.
Reviewed By: yaxunl
Differential Revision: https://reviews.llvm.org/D155870