interp: error location for "unknown GEP" error

This commit removes a panic and replaces it with a proper source
location. The message still isn't very helpful, but at least it points
to a location in the source code.

I'm not very happy with all the `err.Error()` calls, but that's the way
to fit this in a `scanner.Error`. Eventually we should make a
replacement for `scanner.Error` that does proper wrapping of the
original error message.

interp/utils.go

@@ -24,7 +24,11 @@ func getStringBytes(strPtr Value, strLen llvm.Value) []byte {
 	}
 	buf := make([]byte, strLen.ZExtValue())
 	for i := range buf {
-		c := strPtr.GetElementPtr([]uint32{uint32(i)}).Load()
+		gep, err := strPtr.GetElementPtr([]uint32{uint32(i)})
+		if err != nil {
+			panic(err) // TODO
+		}
+		c := gep.Load()
 		buf[i] = byte(c.ZExtValue())
 	}
 	return buf