[clang][Interp] Change array index types in OffsetHelper

This is closer to that the current interpreter does. It also fixes
diagnostics in a case I was looking into. Unfortunately, this is not
possible to test right now since it requires a large array and we
don't implement array fillers yet.

GitOrigin-RevId: 7f0bbbb19b4f13c2efb400db300817aa7ed589cc
1 file changed