Filter out union aliases used for mapping union class names

crrev.com/6250938 introduced an ability to use one of the union aliases
as the class name for the union. However, we also use both the union
class name and all of its aliases to generate member type predicate and
getters for subunions, which would now cause a name conflict.

This CL filters out the name-mapped class name from the list of aliases
for subunions to avoid such conflict.

Bug: 394123703
Change-Id: I88bc851899a25bf034ef209236c9c64d2b2c3e34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6538567
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459519}