PR46255: Fix field diagnostics for C records with anonymous members.

The ParseStructUnionBody function was separately keeping track of the
field decls for historical reasons, however the "ActOn" functions add
the field to the RecordDecl anyway.

The "ParseStructDeclaration" function, which handles parsing fields
didn't have a way of handling what happens on an anonymous field, and
changing it would alter a large amount of objc code, so I chose instead
to implement this by just filling the FieldDecls vector with the actual
FieldDecls that were successfully added to the recorddecl .