[libc++] Remove unnecessary reinterpret_cast from typeinfo

In typeinfo there is a reinterpret_cast between a uintptr_t and size_t. These are two integer types and therefore a reinterpret_cast is not right for this situation. It looks like it may have been copied and pasted from above in the file. An implicit cast works in it's place.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D104814