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Apple-clang complains about missing initializer in union. This fixes it.

Original change's description:
> [base] Use dummy class to avoid superfluous initialization in Optional
>
> The initialization of "char empty_" union-member in Optional is
> redundant and causes the compiler to generate extra code to ensure
> zero-initialization. Using an empty Dummy class for the union member is
> more optimal, since it guarantees that no code for its initialization is
> generated.
>
> Explicitly touching this char member in the constructor also causes a
> problem for conservative GC. Compiler is free to split shared and not
> shared parts of the union in separate memory locations on stack (or keep
> them in different registers). If a conservative GC is triggered at this
> moment, stack scanning may not find the correct object (this is exactly
> what happened in the bug 1044331).
>
> This can be tested in a minimized example: https://godbolt.org/z/t5q5ry.
>
> Bug: 1046776
>
> Change-Id: Idab1a0acf7d7d4f45dc9435a611b23f64e3c3c79
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027948
> Commit-Queue: Anton Bikineev <bikineev@chromium.org>
> Reviewed-by: François Doray <fdoray@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#737494}

Bug: 1046776

Change-Id: I751878a515647e244059d0ad1ba7bc6542fd1f37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033357Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737785}