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
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@228270 138bc75d-0d04-0410-961f-82ee72b054a4