Inplace construct error message in StatusOr while data type is large enough

[PragmaTwice]

We currently use std::unique_ptr<std::string> in StatusOr<T> to represent the error message (in the union type: T or error message), in order to make the error message as small as possible. Hence the size of StatusOr<int> will be equal to 2 * sizeof(void*) rather than sizeof(void*) + sizeof(std::string), where sizeof(std::string) is 32 in libstdc++ (so basically 16 vs 40).
But if sizeof(T) is large than sizeof(std::string) by itself, it is unnecessary to construct the error message on heap for a more compact stack size (since it will never be more compact). Thus we can construct the error message in StatusOr in-place while data type is large enough (i.e. sizeof(T) >= sizeof(std::string)).

[torwig]

LGTM

[PragmaTwice]

Thanks all. Merging...