Use a different way to write StaticAssertExpr() that does not require
the GCC extension statement expressions.
For C, we put the static_assert into a struct. This appears to be a
common approach.
We still need to keep the fallback implementation to support buggy
MSVC < 19.33.
For C++, we put it into a lambda expression. (The C approach doesn't
work; it's not permitted to define a new type inside sizeof.)
Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/5fa3a9f5-eb9a-4408-9baf-403d281f8b10%40eisentraut.org
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