patch 9.1.0496: matched text is highlighted case-sensitively

Problem:  matched text is highlighted case-sensitively
Solution: use MB_STRNICMP, update highlighting when the base changes
          (glepnir)

fixes: #15021
closes: #15023

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/insexpand.c b/src/insexpand.c
index 28e4d34..c673df9 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -1448,7 +1448,7 @@
     char_u *
 ins_compl_leader(void)
 {
-    return compl_leader;
+    return compl_leader != NULL ? compl_leader : compl_orig_text;
 }
 
 /*