@implicitNotFound should emit a compile error when used with non-compile-time constants

[hmemcpy]

I tried adding an @implicitNotFound annotation on a type, and IntelliJ was helpful enough to offer a multiline default with the .stripMargin trick:

@implicitNotFound(
"""Some
    |multiline
    |text
""".stripMargin)

Unfortunately, even though this code compiled fine, the annotation didn't work, I was not able to see the custom message.

Removing the stripMargin code, and turning it into a string literal solved the issue.

I believe this should be treated as a compile-time error, too easy to make this mistake.

[lrytz]

This would be best implemented by fixing #7041

[som-snytt]

Also #7411 for better and more uniform composition.

[SethTisue]

Milestoned for 2.13.6 since, as we discovered at scala/scala#9336, we need to make compiler changes, re-STARR, and then change the annotations to extend @ConstantAnnotation. (And although we can do additional re-STARRs between releases, we prefer not to.)

breadcrumb/note to self