Warn on lossy conversion of literals & constants

[dwijnand]

No description provided.

[SethTisue]

We copied the text of the warning directly from Scala 2.

On further thought, I realized that if you do want the conversion, writing .toFloat or .toDouble, as the warning message suggests, is many more additional characters than simply appending d or f to the constant. Scala 2:

scala 2.13.6> 123456789: Float
              ^
              warning: Widening conversion from Int to Float is deprecated because it loses precision. Write `.toFloat` instead.
val res7: Float = 1.23456792E8

scala 2.13.6> 123456789f
val res8: Float = 1.23456792E8

but as @dwijnand points out, the single character at the end of the constant could be quite easy to miss, so perhaps the recommendation we are making isn't so bad.

No strong opinion on this, but I think the PR is mergeable as-is given that it achieves parity with an already well-established Scala 2 behavior.

[nicolasstucki]

Made test fail in nightly build #13878