Reduce presence of syscache.h in src/include/

ee642cccc4 has added syscache.h in inval.h and objectaddress.h,
enlarging by a lot the footprint of this header, particularly via
objectaddress.h.  A change in syscache.h would cause a lot more files to
be recompiled.

This commit reduces the presence of syscache.h by switching to a direct
use of syscache_ids.h in inval.h and objectaddress.h, where the enum
SysCacheIdentifier is defined.  genbki.pl gains an #ifndef block for
this header, so as its inclusion is more controlled.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/vlcexdcimsmvu3aplt2yxpfndkgtuvjsrms2fdl46rbw3k2kug@drspkoxlaije

src/backend/catalog/genbki.pl

@@ -795,7 +795,10 @@ print $fk_info "};\n\n#endif\t\t\t\t\t\t\t/* SYSTEM_FK_INFO_H */\n";
 # Now generate syscache info
 
 print_boilerplate($syscache_ids_fh, "syscache_ids.h", "SysCache identifiers");
-print $syscache_ids_fh "typedef enum SysCacheIdentifier
+print $syscache_ids_fh "#ifndef SYSCACHE_IDS_H
+#define SYSCACHE_IDS_H
+
+typedef enum SysCacheIdentifier
 {
 \tSYSCACHEID_INVALID = -1,\n";
 
@@ -833,7 +836,10 @@ foreach my $syscache (sort keys %syscaches)
 }
 
 print $syscache_ids_fh "} SysCacheIdentifier;\n";
-print $syscache_ids_fh "#define SysCacheSize ($last_syscache + 1)\n";
+print $syscache_ids_fh "#define SysCacheSize ($last_syscache + 1)\n\n";
+
+# Closing boilerplate for syscache_ids.h
+print $syscache_ids_fh "#endif\t\t\t\t\t\t\t/* SYSCACHE_IDS_H */\n";
 
 print $syscache_info_fh "};\n";
 

src/include/catalog/objectaddress.h

@@ -14,10 +14,10 @@
 #define OBJECTADDRESS_H
 
 #include "access/htup.h"
+#include "catalog/syscache_ids.h"
 #include "nodes/parsenodes.h"
 #include "storage/lockdefs.h"
 #include "utils/relcache.h"
-#include "utils/syscache.h"
 
 /*
  * An ObjectAddress represents a database object of any type.

src/include/utils/inval.h

@@ -15,9 +15,9 @@
 #define INVAL_H
 
 #include "access/htup.h"
+#include "catalog/syscache_ids.h"
 #include "storage/relfilelocator.h"
 #include "utils/relcache.h"
-#include "utils/syscache.h"
 
 extern PGDLLIMPORT int debug_discard_caches;