Puny 24-byte structs are returned by value on SPARC.

Pad these structs up so they are sret-returned even on that
architecture.
This test is passing on SPARC.
Sema: Predefine size_t in MSVC Compatibility mode

MSVC defines size_t without any explicit declarations.  This change
allows us to be compatible with TUs that depend on this declaration
appearing from nowhere.