SILGen: Emit `copy x` on a trivial value as a trivial copy.
Avoids an assertion failure emitting an `explicit_copy_value` on the trivial
value, which is unsupported. This allows `copy x` to compile, albeit with
no effect (which is not ideal, but also not a regression, since no-implicit-copy
controls still don't fully work on trivial values). Fixes #80573 and rdar://148712387.
