x/debug: and support for index and slice expressions.

Slicing an array may yield a type which is not in the DWARF information
of the debugged program, so we add a type sliceOf to handle this.

Change-Id: I79200b036cf183bee7e24cabc5b145e1db522256
Reviewed-on: https://go-review.googlesource.com/16039
Reviewed-by: Dave Day <djd@golang.org>