Fix LLVM assertion on out-of-bounds const slice index.

Fix LLVM assertion on out-of-bounds const slice index.

This turned up as part of #3170.  When constructing an `undef` value to
return in the error case, we were trying to get the element type of the
Rust-level value being indexed instead of the underlying array; when
indexing a slice, that's not an array and the LLVM assertion failure
reflects this.

The regression test is a lightly altered copy of `const-array-oob.rs`.