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

src/include/c.h

@@ -104,6 +104,15 @@
  * XLC: https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.2/com.ibm.xlc131.aix.doc/language_ref/type_attrib.html
  */
 
+/*
+ * For compilers which don't support __has_attribute, we just define
+ * __has_attribute(x) to 0 so that we can define macros for various
+ * __attribute__s more easily below.
+ */
+#ifndef __has_attribute
+#define __has_attribute(attribute) 0
+#endif
+
 /* only GCC supports the unused attribute */
 #ifdef __GNUC__
 #define pg_attribute_unused() __attribute__((unused))
@@ -195,8 +204,6 @@
  * Marking certain functions as "hot" or "cold" can be useful to assist the
  * compiler in arranging the assembly code in a more efficient way.
  */
-#if defined(__has_attribute)
-
 #if __has_attribute (cold)
 #define pg_attribute_cold __attribute__((cold))
 #else
@@ -209,11 +216,6 @@
 #define pg_attribute_hot
 #endif
 
-#else
-#define pg_attribute_hot
-#define pg_attribute_cold
-#endif
-
 /*
  * Mark a point as unreachable in a portable fashion.  This should preferably
  * be something that the compiler understands, to aid code generation.

src/include/utils/elog.h

@@ -160,7 +160,7 @@
 extern bool message_level_is_interesting(int elevel);
 
 extern bool errstart(int elevel, const char *domain);
-extern bool pg_attribute_cold errstart_cold(int elevel, const char *domain);
+extern pg_attribute_cold bool errstart_cold(int elevel, const char *domain);
 extern void errfinish(const char *filename, int lineno, const char *funcname);
 
 extern int	errcode(int sqlerrcode);