[fidl][cpp] Make stubs and proxies always accept generated ordinals.

This allows the stubs and proxies to be resilient to older prebuilts
using explicit ordinals.  We can revert the semantic parts of this
change when everything is using generated ordinals.

This also has a small number of formatting improvements to the generated
code.

FIDL-425

Test: Test added to fidl unittests.
Change-Id: Ic646d9940c6b0276092cd74a27ec376c7417836b
7 files changed