[ObjC] Fix offsets following `[[no_unique_address]]` for `@encode()` (#71321)

Commit 46ca880f made `@encode` skip fields that are made zero-sized by
`[[no_unique_address]]`. When iterating the fields, the index which is
passed to `getFieldOffset` failed to be incremented for those due to the
use of an early `continue`, so subsequent fields reported an incorrect
offset. This caused an assertion to be triggered in
`getObjCEncodingForStructureImpl`.

Fixes #71250