Get pointer to the thrown object if the thrown object type behind the exception is implicitly convertible to the catch type.
There's no saving between phases, so only cache pointer. __cxa_begin_catch expects this to be set.
Convert from pointer to unwindHeader to pointer to CxaExceptionHeader that it is embedded inside of.
Structure of a C++ exception, represented as a C structure. See unwind-cxx.h for the full definition.