[Clang] Fix dependent expression handling for assumptions (#115646)
The function definition instantiation assumes any declarations used
inside are already transformed before transforming the body, so we need
to preserve the transformed expression of CXXAssumeAttr even if it is
not a constant expression. Moreover, the full expression of the
assumption should also entail a potential lambda capture transformation,
hence the call to ActOnFinishFullExpr() after TransformExpr().
Fixes #114787