A composite literal key may not have a global definition, so
Gogo::define_global_names may not see it.  In order to correctly
handle the case in which a predeclared identifier is used as a
composite literal key, do an explicit check of the global namespace.

Test case is https://golang.org/cl/227783.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/227784