Check inline expansion for exclusion

[som-snytt]

Fixes #23018

Preserve attachments on literals, check inline expansions for them, and avoid warning about purity when an inline expansion is a constant unit literal (such as assert(true) and similar reductions).

(The lint for "non-unit value in statement position" specifically does not warn. However, the purity check still warns for explicit parens, since they may have been intended as part of an application.)

Scala 2 warns at refchecks and at that time drills into the tree shapes that may have the attachment.

The test shows status quo, where 42: Unit incorrectly warns, ironically.

[ahoy-jon]

Already 😮! Thank you @som-snytt!

[som-snytt]

In scala 2, these checks are all pushed to refchecks. (I may have intended to follow up with that here.)

Also #20408

[som-snytt]

Saving phase checkWaste for future tweak.

[som-snytt]

This is a good one.

[warn] 282 |    assert(6 * ChunkBits == ScalaFlagEnd)
[warn]     |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[warn]     |    A pure expression does nothing in statement position