Portable StaticAssertExpr

Portable StaticAssertExpr

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