compiler: Accept untyped integral values as string/array indices.

compiler: Accept untyped integral values as string/array indices.

    When determining the type of an index for a string/array indexing
    expression, the gofrontend would disallow floating-point and complex
    values even if they were integral and throw an internal error.  This
    patch changes gofrontend to use an integral type context when
    determining the types of a string/array index.

    Fixes golang/go#11545.

    Reviewed-on: https://go-review.googlesource.com/13796

From-SVN: r228270