WL#720: Add support for [table_name].[column_name]

Add support for syntax: 
 
[table_name].[column_name] 
 
when running in sql_mode=MSSQL or SQL_MODE_QUOTE_WITH_BRACKETS 
 
This is needed to make it easy to port sql code from Microsoft SQL server to  
MySQL. 
 
In other words, there will be three ways to delimit identifiers: 
`with backticks` 
"with quotes" 
[with brackets] 
 
Since we might support arrays someday, we need to decide 
about cases like this: 
SELECT a [5] FROM t 
Is 5 the index of a, or the alias of a?