Problem: Creation of a functional index would not treat column name as
case-insensitive.

Solution: Make the column name comparison case-insensitive when locating
columns in a functional index expression. Specifically, compare column
names using my_strcasecmp(system_charset_info,...) rather than plain
strcmp.

Change-Id: Iadaf2beae98de59091bf8999b6c33d4233c28ee2