[fidl] [llcpp] Fix an undefined behavior in move constructor of unions

A subtle case in the move constructor of llcpp unions was exposed in the
fdio refactor, when the code uses more than two std::moves, and compiled
by GCC for ARM64. The move constructor delegates the move of the union
payload to the corresponding move assignment operator of the payload.
However, the payload's memory is not initilized when its move assignment
operator is invoked, causing potential undefined behaivor.

GCC perhaps took advantage of this and generated unexpected code.

TEST: https://fuchsia-review.googlesource.com/c/fuchsia/+/274654 passes
all the tests, when rebased over this patch. I'm not exactly sure how to
write a test for this exact case.

Change-Id: I05b1eb4be60d921a7ab12f167b45189ab1c4de94