[clang] Update test according to P1937

https://wg21.link/p1937 proposes that in unevaluated contexts, consteval
functions should not be immediately evaluated.
Clang implemented p1937 a while ago, its behavior is correct and the
test needs an update.

Reviewed By: aaron.ballman, shafik

Differential Revision: https://reviews.llvm.org/D145362