After commit 610ec954e1f8 ("[clang] allow const structs/unions/arrays to
be constant expressions for C"), attempts to evaluate
structs/unions/arrays as constants are also performed for C++98 and
C++03.
An assertion was getting tripped up since the potentially-partially
evaluated value was not being reset for those 2 language modes. Make
sure to reset it now for all C++ modes.
Fixes: #65784