[Clang] Loop over FieldDecls instead of all Decls (#89453)

Only FieldDecls are of importance here. A struct defined within another
struct has the same semantics as if it were defined outside of the
struct. So there's no need to look into RecordDecls that aren't a field.

GitOrigin-RevId: 5bcf31ebfad8b32aed20dd47be6238cc19710e63
1 file changed