Warn for insufficient OPAQUE annotations

[DigitalBrains1]

Currently we emit a warning when users forget to annotate their primitive with OPAQUE (or NOINLINE on older GHC's):

Bla2.hs:21:1: Warning: primitive Bla2.bla isn't marked OPAQUE.

We should emit warnings for all cases where people should add OPAQUE: anything annotated Synthesize, including topEntity and testBench; in fact, anything at all where Clash needs to find a name in the Core produced by GHC.