[libc++] Fix a test failure in 7b00e9fae3 (D93815).

"LLVM Buildbot on libcxx-libcxxabi-x86_64-linux-debian" is not happy
with default-initializing the `double` member of `A` in a constexpr
function. At least I'm pretty sure that's what it's complaining about.

GitOrigin-RevId: dd756e3e84812bd962a5b5eaf4f10e9c9338c232
1 file changed