[lworld] RFR: 8341061 [lworld]: Mark and fail compilation in non-discardable values are discarded

[Jesper Steen Møller]

... as per https://bugs.openjdk.org/browse/JDK-8199429

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Commit messages:
 - Merge branch 'openjdk:lworld' into 8341061-lworld-non-discardable-values
 - 8341061: [lworld] value type (re)constructors expressions should not be discardable

Changes: https://git.openjdk.org/valhalla/pull/1273/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1273&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341061
  Stats: 127 lines in 9 files changed: 127 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1273.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1273/head:pull/1273

PR: https://git.openjdk.org/valhalla/pull/1273