pgsql: Tidy up definitions of pg_attribute_hot and pg_attribute_cold

Tidy up definitions of pg_attribute_hot and pg_attribute_cold

1fa22a43a was a quick fix for portability problem I introduced in
697e1d02f.  1fa22a43a adds a few more cases to the preprocessor logic than
I'd have liked.  Andres Freund and Dagfinn Ilmari Mannsåker suggested a
better way to do this.

In passing, also adjust the only current usage of these macros so that the
macro comes before the function's return type in the declaration of the
function.  This now matches what the definition of the function does.

Discussion: https://postgr.es/m/20200625163553.lt6wocbjhklp5pl4@alap3.anarazel.de
Discussion: https://postgr.es/m/87pn43bmok.fsf@wibble.ilmari.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b0727ae99b7056ca88e9708c014a86f6e611d938

Modified Files
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src/include/c.h          | 16 +++++++++-------
src/include/utils/elog.h |  2 +-
2 files changed, 10 insertions(+), 8 deletions(-)