Make fixed-length list building macros work in C++

Make fixed-length list building macros work in C++

Compound literals, as used in pg_list.h for list_makeN(), are not a
C++ feature.  MSVC doesn't accept these.  (GCC and Clang accept them,
but they would warn in -pedantic mode.)  Replace with equivalent
inline functions.  (These are the only instances of compound literals
used in PostgreSQL header files.)

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/CAGECzQR21OnnKiZO_1rLWO0-16kg1JBxnVq-wymYW0-_1cUNtg%40mail.gmail.com