Track span of DISTINCT

[xitep]

Adding span information to the AST node of a DISTINCT will require changing its structure. As per #1548 I'm opening a ticket for this.

Motivation

As described in #1995, or in oracle's docs optimiser hints can be placed right after the SELECT (and before a possible DISTINCT.) (Today, parsing the comment out of the original SQL source can be done manually by consuming the characters right after the SELECT since its AST node has already a select_token telling the keywords ends. Ideally we'd also have the next token's start.)