[Clang importer] Consider attributes on the typedef name for an anonymous tag.

[DougGregor]

In C, one can provide a typedef name for an anonymous tag declaration in
one shot, e.g.,

typedef struct {
  double x, y
} Point;

In this case, there are effectively two declarations at the C level:
the typedef and the struct. The Clang importer was only taking
attributes from the anonymous struct (i.e., the tag) and not from the
typedef. However, any attributes put before the typedef should apply
as well... so consider those, too.

For now, only do this for swift_attr attributes, because we're
seeing this primarily with Sendable annotations. In the future, we
can look to generalizing it, but that could have source-breaking
consequences.

Fixes rdar://91632960.

[DougGregor]

@swift-ci please smoke test