Pass collations to functions in FunctionCallInfoData, not FmgrInfo.

Since collation is effectively an argument, not a property of the function,
FmgrInfo is really the wrong place for it; and this becomes critical in
cases where a cached FmgrInfo is used for varying purposes that might need
different collation settings.  Fix by passing it in FunctionCallInfoData
instead.  In particular this allows a clean fix for bug #5970 (record_cmp
not working).  This requires touching a bit more code than the original
method, but nobody ever thought that collations would not be an invasive
patch...

src/include/nodes/execnodes.h

@@ -864,6 +864,7 @@ typedef struct RowCompareExprState
 	List	   *largs;			/* the left-hand input arguments */
 	List	   *rargs;			/* the right-hand input arguments */
 	FmgrInfo   *funcs;			/* array of comparison function info */
+	Oid		   *collations;		/* array of collations to use */
 } RowCompareExprState;
 
 /* ----------------