[Clang] Fix evaluation context of lambdas appearing in discarded statements (#146857)
Fixes 2 bugs reported in #146063
- The body of a lambda appearing in a discarded statement was sometimes
considered discarded itself
- A lambda conversion operator that was not odr-used was sometimes not
defined even if it was needed
Fixes #146063
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Co-authored-by: Timm Baeder <tbaeder@redhat.com>