This replaces some of the existing checks, and
ensures that we don't emit coverage maps for
property wrapper backing initializers that don't
contain user code.
rdar://99931619
This is the only implicit getter case we care
about, the rest do not contain user-written code.
This means that we now otherwise only ever map
explicit decls, which we need to SILGen anyway. As
such I believe this should fix rdar://39332957.
We do still potentially map implicit expressions,
but should only do so for explicit property
initializers. There is one remaining bug where we
can map an implicit property wrapper backing
initalizer, which I'm tracking in rdar://99931619.