[mlir][OpenACC] Reject unsupported routine bind attributes (#220710)

Require each routine bind item to be a symbol reference or string
attribute. Other successfully parsed attributes left the kind
discriminator uninitialized and were silently dropped.

Found by Coverity.

Assisted-by: Codex
GitOrigin-RevId: 4fbc7d8992dbd21f5a0ea0f09fe647c81b1a8c77