Open up LIKE comparison

Initially, I only allowed LIKE to compare a value to string literal.
However, the right hand side could easily be a parameter, a function
call or concatenated strings.
Default to placeholders in some cases

In cases where it is unclear what an identifier is, don't assume it is a
column name. Instead, only make it a column if it is qualified with the
source name or if there is a single source. Additionally, check for a
prefix to treat an identifier as a placeholder (parameter) rather than a
column.