[Clang] Do not try to diagnose parameter packs in invalid pack expressions (#89257)

In a pack expression, if the id-expression is not valid, do no try to
detect whether it is a pack as that would lead to a crash trying to
print a recovery expression.

Fixes #88929

GitOrigin-RevId: aa22d4422ee031d3867290e6ec12985f87d9ea2f
2 files changed