DRTVWR-575: Introduce llssize (signed size_t) and narrow() function.

llssize is for a function parameter that should accept a size or index
(derived from size_t, which is 64 bits in a 64-bit viewer) but might need to
go negative for flag values. We've historically used S32 for that purpose, but
Xcode 14.1 complains about trying to pass size_t to S32.

narrow() is a template function that casts a wider type (e.g. size_t or
llssize) to a narrower type (e.g. S32 or U32), with validation in
RelWithDebInfo builds. It verifies (using assert()) that the value being
truncated can in fact fit into the target type.