Fix 6461 - multiple definitions with typeid and multiobj

This is an alternate approach to fix http://d.puremagic.com/issues/show_bug.cgi?id=6461
It is competing with pull #2151, but doesn't make all ModuleInfo structs COMDAT causing silent possible failure with duplicate symbols. 
Instead it just emits TypeInfo without "mock" ModuleInfo in case there is no associated module found (usually happening if the backend wants the TypeInfo to be generated).

This patch also reduces the size of the win32 phobos library by about 10%.