Also correct uses of UNIMPLEMENTED() for functionality that is not
advertised as supported, and add spec quotes to justify and document
them.

This change also enforces requiring that the UNIMPLEMENTED() macro's
format string starts with a bug reference of the form b/###. Note we
still have to mention the bug ID in a FIXME() comment as well for
'go/todo' issue tracking (until b/148221114 gets fixed).

Bug: b/131243109
Change-Id: Ieb28e31f2c6c80a27b833626538e7e223b5fda08
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40509
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>