Fix crash-on-valid with consteval temporary construction through list initialization

Clang currently crashes when lowering a consteval list initialization
of a temporary. This is partially working around an issue in the
template instantiation code (TreeTransform::TransformCXXTemporaryObjectExpr())
that does not yet know how to handle list initialization of temporaries
in all cases. However, it's also helping reduce fragility by ensuring
we always have a valid QualType when trying to emit a constant
expression during IR generation.

Fixes #55871

Differential Revision: https://reviews.llvm.org/D131194