We previously allowed *any* conformance constraint to an
ExpressibleBy*Literal protocol to provide a default type (e.g.,
Int/Double/String/etc.) based on the kind of literal protocol. This
led to weird type inference behavior. Restrict the defaulting to
actual literals---not just conformances to literal protocols---which
is a much more reasonable rule.
Because this is a source-breaking change, only introduce this new
behavior when the Swift version >= 4, maintaining the old behavior in
Swift 3 compatibility mode.
