Fix handling of R/W expanded datums that are passed to SQL functions.

fmgr_sql must make expanded-datum arguments read-only, because
it's possible that the function body will pass the argument to
more than one callee function.  If one of those functions takes
the datum's R/W property as license to scribble on it, then later
callees will see an unexpected value, leading to wrong answers.

From a performance standpoint, it'd be nice to skip this in the
common case that the argument value is passed to only one callee.
However, detecting that seems fairly hard, and certainly not
something that I care to attempt in a back-patched bug fix.

Per report from Adam Mackler.  This has been broken since we
invented expanded datums, so back-patch to all supported branches.

Discussion: https://postgr.es/m/WScDU5qfoZ7PB2gXwNqwGGgDPmWzz08VdydcPFLhOwUKZcdWbblbo-0Lku-qhuEiZoXJ82jpiQU4hOjOcrevYEDeoAvz6nR0IU4IHhXnaCA=@mackler.email
Discussion: https://postgr.es/m/187436.1660143060@sss.pgh.pa.us

src/test/regress/sql/create_function_sql.sql

@@ -385,6 +385,19 @@ CREATE FUNCTION voidtest5(a int) RETURNS SETOF VOID LANGUAGE SQL AS
 $$ SELECT generate_series(1, a) $$ STABLE;
 SELECT * FROM voidtest5(3);
 
+-- Regression tests for bugs:
+
+-- Check that arguments that are R/W expanded datums aren't corrupted by
+-- multiple uses.  This test knows that array_append() returns a R/W datum
+-- and will modify a R/W array input in-place.  We use SETOF to prevent
+-- inlining of the SQL function.
+CREATE FUNCTION double_append(anyarray, anyelement) RETURNS SETOF anyarray
+LANGUAGE SQL IMMUTABLE AS
+$$ SELECT array_append($1, $2) || array_append($1, $2) $$;
+
+SELECT double_append(array_append(ARRAY[q1], q2), q3)
+  FROM (VALUES(1,2,3), (4,5,6)) v(q1,q2,q3);
+
 -- Things that shouldn't work:
 
 CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL