Fix arc liberate complaining about concatenation in constant definition

Summary:
Despite constant scalar expressions are supported since PHP 5.6, XHPAST
throws a syntax error. This happens when running arc liberate which in
turn uses XPHAST.
This diffs moves the constant definion line to HEREDOC syntax instead.
It breaks the indentation as indented HEREDOC is only supported from PHP
7.3 which is fairly new and not yet part of most distributions.

Test Plan:
Should return no error:

sudo arc liberate

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2759