At the moment, Clang is rather liberal in assuming that 0 (and by extension unqualified) is always a safe default. This does not work for targets that actually use a different value for the default / generic AS (for example, the SPIRV that obtains from HIPSPV or SYCL). This patch is a first, fairly safe step towards trying to clear things up by querying a modules' default AS from the target, rather than assuming it's 0, alongside fixing a few places where things break / we encode the 0 == DefaultAS assumption. A bunch of existing tests are extended to check for non-zero default AS usage.
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
