Use pg_assume() to avoid compiler warning below exec_set_found()

The warning, visible when building with -O3 and a recent-ish gcc, is due to
gcc not realizing that found is a byvalue type and therefore will never be
interpreted as a varlena type.

Discussion: https://postgr.es/m/3prdb6hkep3duglhsujrn52bkvnlkvhc54fzvph2emrsm4vodl@77yy6j4hkemb
Discussion: https://postgr.es/m/20230316172818.x6375uvheom3ibt2%40awork3.anarazel.de
Discussion: https://postgr.es/m/20240207203138.sknifhlppdtgtxnk%40awork3.anarazel.de

src/pl/plpgsql/src/pl_exec.c

@@ -8610,6 +8610,15 @@ exec_set_found(PLpgSQL_execstate *estate, bool state)
 	PLpgSQL_var *var;
 
 	var = (PLpgSQL_var *) (estate->datums[estate->found_varno]);
+
+	/*
+	 * Use pg_assume() to avoid a spurious warning with some compilers, by
+	 * telling the compiler that the VARATT_IS_EXTERNAL_NON_EXPANDED() branch
+	 * in assign_simple_var() will never be reached when called from here, due
+	 * to "found" being a boolean (i.e. a byvalue type), not a varlena.
+	 */
+	pg_assume(var->datatype->typlen != -1);
+
 	assign_simple_var(estate, var, BoolGetDatum(state), false, false);
 }