Don't treat StoreBorrow addresses as unknown bases. While they are never the base of a formal access, they are returned
as the AccessBase when querying the enclosing scope of an address.
This fixes all analyses that use AccessUtils in the presence of new
forwarding instructions. This is also needed for consistency with the
C++ source base. And for general sanity.
Utilities that walk the use-def chain should never hard-code a list of
opcodes that happened to work with some specific pass. Passes should
never assume that a basic SIL utility handles specific SIL operations
a certain way. Instead, the utility needs to be defined in terms of
SIL semantics. In this case, the utility is supposed to handle all
instructions that semantically forward ownership of a reference.
