Keep softness when widening unions

[mbovel]

Hard unions are union types explicitly written by the user, while soft unions are created by the compiler (for example as the result of a condition or a match). The former should not be widened, while the later should. Currently, this does not work if the union's operands are singletons because the union looses its "hardness" qualification. This PR fixes it.

Before:

val x: Int | String = 3
val y /* : Int | String */ = x
val x2: 3 | "a" = 3
val y2 /* : Matchable */ = x2

After:

val x: Int | String = 3
val y /* : Int | String */ = x
val x2: 3 | "a" = 3
val y2 /* --> : Int | String <-- */ = x2

#14347 and #14360 are linked but have a wider scope: avoiding to widen explicit singleton types altogether. But these are not conclusive yet.