[libc++] Fix a test failure in 7b00e9f (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.

Author: Arthur O'Dwyer

libcxx/test/std/utilities/function.objects/func.memfn/member_data.pass.cpp

@@ -25,7 +25,7 @@ TEST_CONSTEXPR_CXX20 bool
 test(F f)
 {
     {
-    A a;
+    A a = {0.0};
     f(a) = 5;
     assert(a.data_ == 5);
     A* ap = &a;