patch 9.0.0460: loop variable can't be found

Problem:    Loop variable can't be found.
Solution:   Adjust block_id of the loop variable each round.

src/structs.h

@@ -1626,6 +1626,23 @@ typedef enum {
 typedef struct svar_S svar_T;
 
 #if defined(FEAT_EVAL) || defined(PROTO)
+/*
+ * Info used by a ":for" loop.
+ */
+typedef struct
+{
+    int		fi_semicolon;	// TRUE if ending in '; var]'
+    int		fi_varcount;	// nr of variables in the list
+    int		fi_break_count;	// nr of line breaks encountered
+    listwatch_T	fi_lw;		// keep an eye on the item used.
+    list_T	*fi_list;	// list being used
+    int		fi_bi;		// index of blob
+    blob_T	*fi_blob;	// blob being used
+    char_u	*fi_string;	// copy of string being used
+    int		fi_byte_idx;	// byte index in fi_string
+    int		fi_cs_flags;	// cs_flags or'ed together
+} forinfo_T;
+
 typedef struct funccall_S funccall_T;
 
 // values used for "uf_def_status"