Move ExceptionalCondition back to postgres.h

It needs to be defined in the backend even when assertions are not
enabled.  It's cleaner to put it back, than create a separate #ifdef
section in c.h.

Per trouble report from Jeff Janes

src/include/c.h

@@ -619,10 +619,6 @@ typedef NameData *Name;
 #define AssertState(condition) \
 		Trap(!(condition), "BadState")
 
-extern void ExceptionalCondition(const char *conditionName,
-					 const char *errorType,
-			 const char *fileName, int lineNumber) __attribute__((noreturn));
-
 #endif /* USE_ASSERT_CHECKING && !FRONTEND */
 
 

src/include/postgres.h

@@ -639,9 +639,15 @@ extern Datum Float8GetDatum(float8 X);
  */
 
 /*
- * This declaration supports the assertion-related macros in c.h.  This is here
- * because that file doesn't have PGDLLIMPORT in the right place.
+ * These declarations supports the assertion-related macros in c.h.
+ * assert_enabled is here because that file doesn't have PGDLLIMPORT in the
+ * right place, and ExceptionalCondition must be present, for the backend only,
+ * even when assertions are not enabled.
  */
 extern PGDLLIMPORT bool assert_enabled;
 
+extern void ExceptionalCondition(const char *conditionName,
+					 const char *errorType,
+			 const char *fileName, int lineNumber) __attribute__((noreturn));
+
 #endif   /* POSTGRES_H */