RTS: Fix CloneStack.c when no table next to code

Function `lookupIPE` does not modify its argument. Reflect this in the
type. Module `CloneStack.c` relies on this for RTS without tables
next to code.

Fixes #20879

rts/IPE.c

@@ -157,7 +157,7 @@ void registerInfoProvList(InfoProvEnt **ent_list) {
     RELEASE_LOCK(&ipeMapLock);
 }
 
-InfoProvEnt *lookupIPE(StgInfoTable *info) {
+InfoProvEnt *lookupIPE(const StgInfoTable *info) {
     updateIpeMap();
     return lookupHashTable(ipeMap, (StgWord)info);
 }

rts/include/rts/IPE.h

@@ -28,4 +28,4 @@ typedef struct InfoProvEnt_ {
 } InfoProvEnt;
 
 void registerInfoProvList(InfoProvEnt **cc_list);
-InfoProvEnt *lookupIPE(StgInfoTable *info);
+InfoProvEnt *lookupIPE(const StgInfoTable *info);