[ObjC++] Never pass structs that transitively contain __weak fields in

registers.

This patch fixes a bug in r328731 that caused structs transitively
containing __weak fields to be passed in registers. The patch replaces
the flag RecordDecl::CanPassInRegisters with a 2-bit enum that indicates
whether the struct or structs containing the struct are forced to be
passed indirectly.

rdar://problem/39194693

llvm-svn: 329617