If `buildBlock` is also unavailable, or the
builder itself is unavailable, continue to solve
using `buildPartialBlock` to get better
diagnostics.
This behavior technically differs from what is
specified in SE-0348, but only affects the invalid
case where no builder methods are available to use.
In particular, this improves diagnostics for
RegexComponentBuilder when the deployment target
is too low. Previously we would try to solve using
`buildBlock` (as `buildPartialBlock` is unavailable),
but RegexComponentBuilder only defines `buildBlock`
for the empty body case, leading to unhelpful
diagnostics that ultimately preferred not to use
the result builder at all.
rdar://97533700
