Lint runs before purity

[som-snytt]

Inspired by #16115

Run lint before value discard before purity check.

Just adding -Wvalue-discard provides the requested warning from the ticket.

There are two discard messages:

-- [E175] Potential Issue Warning: i16115.scala:13:4 -------------------------------------------------------------------
13 |  C(K),
   |    ^
   |    discarded non-Unit value of type (E.K : E)

for discard, as opposed to the purity check

-- [E190] Potential Issue Warning: i16115.scala:13:4 -------------------------------------------------------------------
13 |  C(K),
   |    ^
   |    Discarded non-Unit value of type E. You may want to use `()`.
   |
   | longer explanation available when compiling with `-explain`

which could be made uniform.

The message for -Wnonunit-statement is noncommittal:

-- [E176] Potential Issue Warning: i16115.scala:13:4 -------------------------------------------------------------------
13 |  C(K),
   |    ^
   |    unused value of type (E.K : E)

[som-snytt]

Suspiciously, no check files required updating.