compiler: don't name type descriptor for alias type

The test case is https://golang.org/cl/275632.

Change-Id: Ibbbdc84f0e2151fb938de8fea7021be8cd26c703
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/275652
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>