Support indexing on expressions and functions for the MySQL dialect

Support indexing on expressions and functions for the MySQL dialect

A user noticed that creating an index where the "key part" was an expression
or function would raise an error for MySQL because the key part was not
parenthesized. The proposed change will check whether a key part is not a
Column or Unary Expression  and parenthesize if the case is False.

This fix also contains a minor fix to a test case that was previously incorrect
(`def test_create_index_expr():`).

**Have a nice day!**
Fixes: #5462
Closes: #5587
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5587
Pull-request-sha: 7515e50cd7435744fc79c210b2f3aa4c0546ba28

Change-Id: Id1b3b3026983c0e05808baa243e354f82b78180c