Fix reinterpret cast warning of unsigned to void* in Metdata.h

[hughbe]

warning C4312: 'reinterpret_cast': conversion from 'unsigned int' to 'void *' of greater size

Instead, use uintptr_t, since this should be the size of void *

[gparker42]

Looks good. I was suspicious about using uintptr_t instead of intptr_t because index is signed, but swift_getFunctionPointerExtraInhabitantIndex also uses uintptr_t and clearly does not want sign extension.

[gparker42]

@swift-ci please smoke test