[clang][Interp] Classify ArrayInitIndexExpr type

We can't just push a uint64 unconditionally here, since on 32bit arches
we later expect a different type, e.g. uint32.

This broke e.g. these builders:
https://lab.llvm.org/buildbot#builders/171/builds/21514
https://lab.llvm.org/buildbot#builders/38/builds/6643