[NFC] [AST] Reduce the size of TemplateParmPosition

I found this when reading the codes. I think it makes sense to reduce
the space for TemplateParmPosition. It is hard to image the depth of
template parameter is larger than 2^20 and the index is larger than
2^12. So I think the patch might be reasonable.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D123298